Right now planetary invasion is nothing more than a place holder for a future system, one of which I am very excited to see when finally finished.  However, I'm worried..

Planetary invasion has always been something space 4x developers have such a hard time handling.  They either outright ignore it (easily half of the  space 4X titles) make it too simple or have little meaning with very little involvement (galciv II, ) or make it so unintuitive and complex the AI and player can barely deal with it (forgot the titles name but there are a few I've experienced..)

So to in order to create a great expansion and a meaningful, immersive invasion mechanic I think taking a more involved, multi-turn approach involving the creation of supply lines, espionage and counter espionage, assualt and defense tactics and attruition without turning it into a micromanagement or developer coding nightmare, since a lot of it will be handled in a very similar fashion to how trade routes and events work.

First you'd destroy the fleets protecting the planet and park your fleet on the planet to create a sort of blockade.  Then you'd select a planet you'd like to stage the invasion efforts at (multiple planets/transports supporting the invasion effort would make the assualt take less turns/be more effective and without causing too much of a drain on each planets individual pop) then you'd build a troop transport then fly the ship to the planet. Once there it will create a supply line, transporting troops and supplies to and from the supporting planet until its conquered, then it will be consumed.  This supply line can be raided and the troop ships along it can be attacked to hault/weaken the invasion effort and begin draining on the attacking forces.  

Basically each turn as long as there is a protected supply line to the invaded planet, enough funds in the treasury (each troop transport supporting an invasion will create an invasion expense, increasing over time, kinda like the opposite of a trade route, encouraging efforts to finish the invasion as quickly as possible.) Sufficient population on the supporting planet, and a fleet of friendly ships in orbit above the besieged planet a progress bar will gradually fill up, which represents the forces and resistance attruition.  This can take many turns or a couple turns depending on the planets pop, improvements, natural planetary features, if its a home world, and the amount of planets contributing to the fight, ideology's, tech, senate laws, events etc.

Along with this, each turn you and the defending player will be required to select a single tactic for each invasion taking place.  This is where the player becomes directly involved in the assualt of the planet.  Depending on the tactics chosen by the two combatant empires determines just how much progress is made towards the victory of the planet for the attackers.  For each attack tactic there will be a specific defense tactic that can totally stall progress for that turn. Every other defense tactic will either decrease that turns progress by a certain percentage based on some random factors or not cause any effect, once again Dependant on the chosen attack tactic, giving that turn the maximum progress it can make towards victory for the attackers.  Though a defending tactic can never negatively decrease the total progress made.  Only by cutting off the supply lines, destroyong the blockading fleet, or the enemy becoming bankrupt can an assault's progress start to decrease until the invasion fails.  

If the defending player can defeat the blockading fleet and protect the planet long enough they can start sending troop transports and create supply lines to the besieged planet too to further increase the speed the attackers siege progress falls until the invading forces are eliminated totally, creating more supply lines to speed it up even more just like attacking.  Since as long as there are invading forces on the planet there will be no production for that planet, a large negative moral effect, and the chance that the enemy will take back control of orbit, resuming the siege progress where they picked it back up at

But wait there's more!  Assuming spying will come along with proper ground invasion and it being done in a similar fashion as galciv 2 (it was pretty good) infiltrating the enemies empire with spy's increases the odds of you automatically doing maximum progress no matter what each of you pick for that turn (simulating you discovering the enemies plans and immediately changing tactics to counter.). This will happen at random normally since secrets are hard to keep regardless but the chance is very low without totally maxing out the espionage spy's for that empire, and next to none if you don't have ANY spy's on them.  But for spy's to be truly effective, you'd want to place them on the beseiged planet itself, or if your the defender, on one of the enemy's supporting planets.  This way you increase the odds of automatically countering the tactic up to 50%, until the spy is nullified by another spy.  All this adding another layer to the invasion of enemy worlds by needing to keep tabs on enemy espionage and keep yours well funded.

Considering all of this, and if it can be done this way, I can easily see it as being one of the most interesting invasion features of any space 4x game to date without making it too simple but still making it exciting, involved and deep without becoming too much to manage, for both the AI and player.  Weeding out spy's, raiding supply lines, issuing tactics, choosing supporting planets, and trying to keep up with the incredible expenses can make for a truly immersive, dramatic, and eventful wargame that actually has a real sense of being in a grand war beyond what takes place in space.  I can already think of experiences player can have with such a system.  Choosing just the right defense tactic a turn away ffrom defeat, giving just enough time for your attack fleet to clear the blockade turning the tables on an exhausting 2 year siege.