Ok, first off I need to make it clear that I do not play multiplayer so these changes are only going to be related to singleplayer.
First I would suggest more options, so that these things that would improve singleplayer play could be altered/turned off for multiplayer. This will make both sides happy hopefully.
AI types - I have a real hard time telling the difference between the aggressive, fortifier, researcher and economist AIs. I have been in the habit of watching the AI progress in my games as of late. Every 30 minutes or so I save the game and exit and check out how each AI is doing in various stats, credit income, metal/crystal, total research done etc.
I often find that the AI are not doing what their name suggests. Often the fortifier AI is out researching the research AI by an incredible margin, while in the actual game they are doing relativly well(so its not just that the research AI is half as powerful as the fortifier AI)
So to fix this here is what I would suggest.
The aggressive AI - It should do many more small raids. Sending in small groups of frigates to attack undefended metal and crystal extractors would really help. Currently the AI builds up a large force before attacking and is easily countered by forming your own large fleet. If the AI was able to do multiple hit and run raids it would be much more "aggressive". The AI should not only make attacks on the frontline planets but if it finds a weak planet deeper into enemy territory it should send its fleet there, or a hit and run raid. It may lose some ships going through enemy frontline planets but this will force the enemy to defend all planets not just the 'bottlenecks'.
The Fortifier AI - Well, this AI should fortify its planets. I rarely EVER see any of the AI building turret or hanger defenses. I would like to see the AI max out its planets with defenses much more often. Not only the fortifier AI but all the AI need to utilize these defenses more. Currently it is easy to swoop into an enemy planet and destroy all orbital structures before the AI is able to rally defenses. Oribtal defenses would at least provide a buffer while the AI can get ships in there.
Research AI - It needs to do more research. It should limit its fleet size to save income for research. Keeping a smaller higher tech fleet will give it more power in the late game. The research AI should move quickly to go after the high powered research object. For example, a Vasari research AI should move very quickly to get Returning Armada. It should keep its fleet population 'low' to have high income, once reaching returning armada it should expand its fleet cap to create an explosive fleet.
Economist AI - Not sure what to say about this AI, I find it does no better at making credits and resources than any other AI. It should be investing a heavy portion of its resources into building trade ports and refineries and researching items which improve its economy. It should also focus heavily on capturing the 'prebuilt' extractors. I am often able to quickly capture the extractors near the enemy home systems and it rarely ever tries to capture them back. The economy AI should try to maintain as many of the capturable extractors as possible, and defending them with small fleets to protect against enemy efforts to capture them. Doesn't need to repel enemy fleet, but repel lone ships sent to capture them should be easy.
Further thoughts on the AI weaknesses.
The easiest strategy to beat the AI, that I have found so far, is to maintain a very small fleet. This allows me to have a great income. Using this income I max out all planets with trade ports and refineries which further increases my income. At the same time I am investing massive amounts of resources into research, this allows my small fleet to repel the larger AI mob of junk ships. Playing as Vasari, I fortify all planets with maxed out tactical slots, using 1 phase stabilizer, 1-2 hanger defense, and lots of missile turrets. This provides plenty of buffer against the AI, if they attack, my small defense force uses the phase stabilizer to get there in time before anything major as been destroyed. Using scouts I am easily able to find planet the enemy has not bothered to defend and use a small strike force to take it over. The AI rarely makes a defense, or if it does, it is far too late.
Problems all the AI have - Lack of planet development. Both planetary upgrades(population, planet health, etc) and orbital structures. Late game there is no reason the AI should not have all of its main planets fully maxed out on planet upgrades, logistical structures and tactical structures.
AI increases its fleet size too early in the game, this really dampens the AI's income and screws it over.
All AI need to try to claim capturable extractors.
All AI need to spend more resources on research.
All AI need to build more orbital defenses.
In late game all AI should spend credits to level its capital ships up. It should have the credit income late game to be able to afford this easily, they are too worthless late game as level 1, they often die before they reach level 2 or 3.
When AI is attacking enemy planet it should not just attack the planet in the shortest distance possible. Currently you only need to place defenses in the area of the planet the phase lane is on. The AI should be able to skirt the perimeter of the gravity well and siege the planet from the least defended side. If there is no 'least defended side' the AI should attempt to strike vunerable targets such as trade ports/extractors/refineries. Phase stabilizers should be esspecially priortized targets as they will prevent defense fleets from arriving easily. Currently the AI rarely will attack the phase stabilizers which allows ample time to get a defense fleet over to the planet.
In the event of a planet being heavily defended by hanger defense, the AI should have all bombers dock and use only fighters and flak frigates until most enemy fighters are destroyed.
My suggestions to change items in the game beyond the AI.
Increase HP of planetary defenses. Not by too much, I would say give them shield systems. This would give them some more flavor as well as each race would get its hull/shield appropriate ratios. TEC would heave weak shields but heavy hull, advent heavy shield weaker hull. Total HP could remain the same as now, the shield mitigation would be the 'boost'.
Reduce fleet upkeep. The amount of income sucked up by fleet upkeep at the high levels is really quite silly. Especially for the AI. Reducing the upkeep would help the AI not get killed by a player who keeps his fleet small and highly researched. Alternativly, base upkeep off of USED ship slots rather than total ship slots. This way when the AI has his fleet wiped out, his income increases to rebuild, and will not penalty the AI for upgrading his fleet cap early on, as long as the AI doesn't spam tons of ships.
Fix the black market. Currently it is too cheap to buy resources. Even the 'boom' prices are jokingly cheap to buy and silly cheap to sell. Booms should allow prices to reach 2000 buy and 1500 sell. AI needs to play better with the market than it currently does.
There needs to be a planetary defense structure that is able to kill strike craft. Hanger defenses are less than effective at this. It is so easy to keep your strike craft docked in their carrier ships and let your flak frigates kill all hanger defense fighters/bombers, then unleash your strike craft to destroy enemy hanger defense/turret defenses. Turret batteries should either have their own flak mounted on them, or an entire new unit. The Flak turret can be created to protect against his threat.
Current turret defenses should be set as frigate defenses and a new turret defense for engaging larger targets created. They would be unable to attack the smaller ships but would do good damage to capitals and cruisers.
The reason for changing these turrets is that currently you can just spam the turret defenses on the side of the planet the AI will come in at and they will chew them up too easily.
That is all I have for now. I would like to state once again that for ALL of these changes I have NOT FOR A MOMENT considered player vs player gameplay. I believe player vs AI and player vs player gameplay is SO VASTLY DIFFERENT that there really should be two sets of gameplay rules for each one. In order to have a great Player vs AI gameplay you will probably have to have a not so hot Player vs Player. If there was a sort of option to distinguish the two game balances it would be fantastic.
Perhaps an option as "Competitive Play" which would enable a mod that was balanced for player vs player and took no consideration for what an AI requires to be challenging. This would give those people who love the online aspect of Sins of a Solar Empire the chance to have a game that was fantasically balanced for their gameplay while leaving room for those who only care about single player to have gameplay that makes the AI a real enjoyment to play.
I believe that attempting to create a game balance that gives both a challenging AI and great player vs player gameplay is not going to happen without major breakthroughs in AI programming, I believe the answer lies in seperating player vs AI and player vs player gameplay.
Comments/suggestions are welcome, but please do not turn this thread into a flame war. If you share a difference of opinion, by all means state it. But give your intelligent reasons for why, rather than just say it is a bad idea and calling people names. Also I know many of you will cringe at these ideas because you will think of them in the player vs player context but remember, I am not suggesting any of this be forced upon you in player vs player play.