Sins focus = space. Yes! Good... In a "perfect" world we'd have it all... and a toaster... and directional armor, etc., but I'm happy to see Sins focus on key strengths. That said, I couldn't help having some fun...
Conventional Combat Integration Method
Objective: Retain space focus of the game, do not overtax player attention / resources by engaging them "on planet", do not overtax low-end computers running sins, enhance gameplay experience, do not require major substantial engine changes, etc.
Based on that, I broke it down into five subjects, which might do it:
PLANETARY UPGRADES; generic "Upgrade Defenses" + tech tree aspects.
ASSAULT SHIPS (as differentiated from long-range weapons ships): concept examples.
CONVENTIONAL COMBAT MECHANICS (resolved "off screen"): worked only in principle.
ANNEXATION & ASSIMILATION You've got a planet! What do you do with it. Or did your really want it?
CONVENTIONAL COMBAT TECH TREE Various example techs of basic branches listed.
Note: I'm not suggesting Ironclad or tired modders do this... if it gives you cool ideas, I'd be delighted, but I like thought experiment. Respect to the all your hard work, as is.
PLANETARY UPGRADES: Defensive Forces.
Like the other upgrades, this would be a "generic" green button. It will generate defensive forces according to your planet's population and tech level. To get more special abilities, rather than building any planetary structures, you just purchase the tech.
Uprade, per level: defensive forces = to increasing % of population.
Bonus for High Security Protocol
Type of Forces & Abilities: See Tech Tree.
SHIPS
FRIGATE RAIDER
(TEC Example): Frigate designed for light raiding, suitable for asteroids or *very* lightly populated planets. If used on asteroid, remains in existence. If used on planet, is consumed (once you drop it to the surface in a gravity well that big, it ain't coming back out). In either case, the ship itself is the troop delivery vehicle.
ARMAMENT: Non or a few light autocannons; nominally to help with fighter-defense and provide a little close fire-support. Fairly negligable.
UPGRADES: Fast Raid (gets to the target faster), Grapples (can be used to latch onto an enemy construction craft, and reel the ships together, allowing a hostile takeover of construction craft.
CRUISER: EXPEDITIONARY ASSAULT SHIP
(TEC Example): A large cruiser which carries a battalion-scale element, with vehicles & support, and delivers them via company-scale dropships. Dropships are launched at planetary bombardment range, and must fly to the planet with little avatars alike to frigates. During this phase, hostile ships present could fire on them.
CRUISER ARMAMENT: Flak
UPGRADES: Pinpoint Ortillery, Drop Pods, Theater Command, Grav Armor, etc.
Why Not a Cap? Dedicating a capital ship to ground assault / infantry makes little sense, as well as detracting from the space-battle-focus of Sins. In real life, assault ships aren't "capital" ships either. The assault cruiser should remain a support ship... which also grants greater flexibility in force composition, and does not detract from the "battlefield centerpiece" role of Caps in Sins.
BASIC MECHANICS
Conventional Combat Units can be divided into 5 primary categories:
1. INFANTRY
2. ARMOR
3. ARTILLERY
4. AERIAL
5. FORTIFICATIONS.
The basic objective of course would be to enhance the playing experience, so probably the best way to do this would be to have a complex countering system (as Sins has for space combat), make it possible to gain some pre-intelligence about your foe (otherwise what you've readied is a total crap shoot... not sure how you'd handle that), and give different units different planetary affinities, etc (which might handle the former question quite well).
EX: "Desert worlds have good atmospheric conditions and open terrain; that means Aerials and Armor have the advantage. Maneuver rules the day. Expect a lot of both going in."
Most streamlined way to handle it might be of course to have the computer resolve combat, and display a simple pop-up with ongoing planetary combat results.
COUNTERS & AFFINITIES:
INFANTRY: Backbone force. Counter: Artillery & Armor. Infantry is strong vs. Infantry, Planet Aspects, and Special. Affinity: Home Planet Type. Infantry is only force that can actually take and hold ground a planet. If you have no infantry left, you may beat down the defenses, but you can't "take possession."
Special Tech: STA (Surveilance and Target Acquisition, to borrow the USMC Acronym). STA troops are stealth recon operators who can provide targeting information, improving hit rates & effective countering. Within the Sins context, they use things like chameleonic suits, relay ortillery targets, and tag enemy craft with long-range target designating rounds for homing missiles.
Special Tech: Saboteurs (Assault). Specialize in attacking enemy infrastructure.
Special Tech: PsyOps, etc. possible.
ARMOR: Counters: Artillery, Aerial. Affinity: Open terrain worlds - Dessert, Ice. [Unless you implement a feature of "Crevasses" or "Sand in the Treads" which will make armor sad.] Armor excels where it can move fast and hit hard, outrunning its counters or their firing solutions & closing to favored engagement ranges.
Special Tech: Grav Engines. Provides "Hover" armor, with increased mobility. Negates some terrain "bog down" problems.
Special Tech: Networked Gunnery. Improves effectiveness via networking with other vehicles and ground troops.
Special Tech: Cavalry Rush. Upgrade to Grav Engines; large propulsion powerplants provide almost aircraft-like speeds to armor, assisting in evasion & rush assaults.
ARTILLERY: Counters: other Artillery, sometimes Armor, other Aerial. Affinity: Rough terrain worlds - Volcanic, Terran. [Terrain slows mobility, the bane of artillery]. Artillery, when well supported and in affinity terrain, tends to inflict the majority of death and destruction, but unprotected or in non-affinity terrain, it fairs not so well.
Special Tech: Counterbattery. Aids vs. other Artillery.
Special Tech: Smart Warheads. Aids vs. Armor, by providing intelligent seeker warheads.
Special Tech: Strato-Flak. A nickname for HV artillery that puts shells very high in the atmosphere, where they burst and tear apart incoming drop pods with immense volumes of very fine, hypervelocity shrapnel - the idea being that, at the speeds and temperatures the pods are moving, it takes very little to make a whole lot of unhappiness.
Special Tech: Fire Haven. Self-forming networks of in-flight smartshells, close weapon systems, and artillery batteries co-ordinate to create "fire havens," zones often recognizable only by the dome of mutually annihilating ordnance surrounding them. Fire Havens have the potential to protect not only their inhabitants, but extend outwards in detached hemispheres or globes of active protection. + to Protection, can establish mobile fire havens around advances. Particularly potent in combination with high-speed Armor.
AERIALS: Counters: Special (AA), other Aerial. Affinity: Anyplace with good flying conditions. However, all units get a bonus vs. Aerial attack on Terrain worlds (due to vegetative cover, which makes targeting harder for the Aerials) or Volcanic worlds (all the crap & fume in the air obstructs long-range visibility, etc.)
The standard Aerial unit is
multi-role interceptor. Unlike space strike craft, aerial strike craft are tailor-built for atmospheric operation.
Special Tech: Knifefighters. 50% of standard Interceptors are upgraded into high performance air-superiority craft. Hard Counter: Aerials.
Special Tech: Gunships. 25% of standard Interceptors are upgraded with higher performance anti-infantry weapons and given a Close Air Support bonus. Hard Counter: Infantry.
Special Tech: Interdictors. 25% of standard Interceptors are upgraded with improved armor-piercing missiles and and low-level, high-velocity flight avionics. Hard Counter: Armor, Arillery.
Special Tech: Vertical Intercept. Aerials are improved for mid-descent launch and engagement (assault) or vertical, high-altitude attack (defense). The objective on the defense side is to bring more weapons to bear on an invasion at one if its most vulnerable points (atmospheric entry); the objective for the assault is to counter that action, and/or distribute craft early to avoid losses of strike craft if a dropship goes down.
FORTIFICATIONS: This is a bit of a pseudo-category... perhaps a way to quantify the "dug-in-ness" of defenders, or the advantages of - say - a dropship whose armor and heat shields reconfigure on landing into blast shields to protect a central artillery battery. Realistic? Who knows... but we're fighting Jutland In Space, so "cool" is reason enough. 
So, mechanics-wise... [Units + their techs/upgrades + other modifiers (planetary tech, terrain, strategy tech, etc.)] vs. [same for opposition forces] leads to an autoresolution. Now you've captured a planet...
ASSIMILATION & ANNEXATION
The objective of seizing a planet, as opposed to bombing the ever****** **** **** **** **** *********t out of it is to seize infrastructure and/or inhabitants intact. Infrastructure is instantly seized. However, productivity is penalized until the inhabitants are assimilated. Fortunately, Ironclad gave us a pre-existing tool for this: Culture. If the player's culture is dominating (their "culture influence" is strongest on that planet), assimilation occurs at a rate of X/second, calculated off of the amount/degree of cultural dominance. Cap ships are of course factored in when present.
Obviously in a "zero culture" zone, there is no real change... the planet won't assimilate, but won't rebel and try to go neutral either. It just kinda sits there. Fortunately, in a "nil zone," if you can keep you cap ship(s) parked in orbit, you'll keep assimilating, since they provide cultural influence.
However, if another culture is dominant at the location, the population will not assimilate. You might as well just bomb them: as long as another culture dominates, the planetary population will exist in a state of insurgency, and productivity of the planetary system will be zero.
CONVENTIONAL COMBAT TECH TREE
I'll divide this into branches, each delineated by a green header. Omitting every tech mentioned previously, under the specific unity groups. (Consider it all just a branch "unit tech"). These are just some (sort of random) ideas; use your imagination.
THEATER COMMAND: The cruiser hosts a special ground-combat CIC. Ship-mounted sensors, rapid-dispersal microsatellites, and a commo/sensor network with ground troops and dropships allow the cruiser to co-ordinate intelligence and operations for the ground war.
ECM: Massive electronic countermeasures are brought to bear on the planet in an attempt to destroy the communication and co-ordination of defensive forces.
PRE-BARRAGE: Theater Command is better at co-ordinating pre-barrage of a planet with other vessels, directing the otherwise "meaty fist" of siege craft to hit select strategic targets, leaving most of the population alive and perhaps the civil infrastructure intact.
PINPOINT ORTILLERY: Assault Cruisers do not prrovide massive "pre-bombardment"; those services are rendered by siege frigates or cap ships; most space on assault ships is dedicated to their assault forces, delivery systems, and supply. However, an upgrade above Theater Command provides batteries of highly-accurized kinetic kill weaponry, capable of being guided all the way to impact, and hence used in close proximity to ground forces. (Think "a chunk of tungsten rebar with actuated fins, falling from low orbit.") PO assists ground forces directly.
PLANETARY INVASION:
ARMORED DROPSHIPS: Dropships use some variant mixture of energy shields, ablative shielding, and massive slab of armor on their belly to mitigate re-entry. Armored Dropships up-armor against ground-based fire at this, their most vulnerable stage, when their signature in unmissable and their trajectory is basically set until they hit "thick air."
DROP PODS: Standard assault mechanism for Assault Carriers is high-velocity dropships, each usually carrying a platoon or even company-size element, essentially short-range, less expensive equivalents to the assault frigate. This is generally advantageous because a drop ship can be more heavily armored than a drop pod, and delivers larger units intact to the battlefield.
Drop pods require development because - though the actual construction of a pod for a single soldier, fire team, or even squad is technically quite manageable, stealthing the signature of atmospheric reentry vehicles is extremely difficult. As such, small drop pods tend to fair much more poorly against ground-based AA defenses than dropships. Drop pods require the development of hardened pods and substantial ECM. They are considered most effective in combination with special operations forces... otherwise you're just scattering you forces all over an area.
HYPERVELOCITY SAM SILOS: HV Surface-to-air missiles intended to engage dropships high in the atmosphere, during their most vulnerable stage.
KINETIC DEFENDERS: Guided Kinetic-Kill weaponry that rides down into a gravity well with dropships, seeking to take out SAMs and other upward-coming weapons in suicide attacks, defending the dropships.
FLEET ASSETS
FLEET ARMSMEN: I like this branch of the tree because it focuses on Sins great strength: Space. All ships get a security complement, according to size. These Armsmen can do microgravity raids (asteroids; you'll need to "park" next it) and seize structures. They can also board immobile ships or repel boarders (another ball of wax and set of rules, weapons, etc... sorry, it was the MoO2, not me, talking.)
Next Tier: VAC COMMANDOS: EVA-specialized elite Armsmen. Vac commandoes aren't of much use in battle ("sir, you want me to go outside the hull NOW?") but can cross open space and potentially board moving vessels, like trade ships or refinery ships, to capture them, or cut into structures. Vac Commandoes are something you'd run a scout or light frigate, etc. alongside such ships to "seize" the prize. Although it would be wonderful poetry if you captured a pirate ship. Speaking of which, pirate ships don't have shields, do they...?
Next Tier: DENIABLE ASSETS: Mercenary or Bionic Vac Commandoes who aren't traceable back to your empire. Run a scout through an Ally's grav well or an asteroid field, drop a sleeper group of 'em, wait a little while, and the prizes start sailing into port... or that laboratory suffers a mysterious explosion.
Next Tier: ANTIMATTER BOMB: Isn't this a Scout Power? Well... yes. But I like the idea of it being done by a group of heavily tattoed VACies bailing out of a tiny ship, and patiently free-falling across 120,000 klicks with satchel charges of antimatter and profound social dysfunctions.
STRATEGY & TACTICS: Because conventional warfare is auto-resolved, player skill does not enter. Therefore, various levels of tech can represent sophistocation in this area. Example possibilities:
PROFESSIONAL ARMY: You have moved beyond conscription; all troops are professional volunteers. + (x) to effetiveness.
STRATCOM: HQ is well integrated with field units; sub-commanders have reasonable autonomy and contingency training. Your conventional war apparatus functions well at a strategic level. All units gain + (x) effectiveness.
STRATEGIC AI: AIs augment your decision makers and manage resources and logistics at higher efficiency.
PLANETARY RANGERS: All infantry are upgraded to "elite" status, thanks to improved recruitment, training, health care, etc.
COMBAT SPUs: Vehicles are equipped with AI augmentation or, in some cases, fully autonomous drones. +(x).
etc.
Well, that was kind of a... tome [edit change: "no, it is a Monster."] Realistic? No... but I had fun! Time for beer.
