For one thing if the player puts a suped up starbase near you to attack. Don't attack it over, and over. Remember some races start out with a tech where they can see all the stars. If they have that they can at least see where to send colony ships as far as they know a star is there. I check out other people's tech through diplomacy, and espionage. I like how the korath identified what kind of warship to build against you by what kind of warship you use in dark avatar.
The only problem they needed to be quicker. The annoying savatoge of the iconians, and krynn in da was good. The only real imfastructure problem I've seen was you didn't use bonus tiles right if you have a certain bonus use it. Lets not put a starport on a bonus tiles. You really need to work your manufacturing and technology capitals on research, and production bonus bigger planets. You need to research more smaller bonus techs.
Some ways the terrans and altarians did better infastracture better in dark avatar. The krynn did good influence in dark avatar. Iconians had decent war stradegy as far as planetary invasion in dread lords. A real problem I've seen they would have hundreds, or thousands of bc's they wouldn't use, or they would be broke. For the most bc's are better spent in production, or colony ships instead of anywhere else. I did miss the large fleets of the drengin, korath, and krynn from da.
I don't like bad guy's to weak to bother anyone. I liked the thalans and dread lords by the way. I don't like nerfed expansions. You appearently have an exploit where if you keep even research, production, farming, and econimy is the most preferable untill you reach 40 billion population it is most preferable. Fix this or use it.
On upgrading only do that if your income is better than your expences. Only upgrade tiles if you don't have anymore empty ones. Identify as far as ships go what things you have from techs vs empty space to use effectively, and upgrade when you can. Don't fall so much in debt you have to shut down your empire, even if this means you have lower your production, or tear down some buildings. If you are not using some spys use them.
I don't see unarmed defender's defending planets as a problem. The problem is they should've had weapons. By the way declaring war to early worked untill I figured out the ruse. Remember if you don't fix the exploit building the technological, and manufacturing capital together helps as your technological capital. Just make sure that the ai can play its stradegy ie yor.
I hope these tips wIll help you out. Don't have nerfed colonization races, or make sure everyone builds imfastructure. As far as the jagged knife goes they need to do what it takes to not go broke. They also need to build all their super projects left. Ways to fix jagged knife, build economic structures rush buy if possible.
Sell some of their more expensive buildings to maintain. My reasoning for using their super projects is to make retaking them sweeter. The other ai's will have a combination of wanting their planets back to wanting to take the best planets from the jagged knife. The way to identify best planets is by class unless you have the espionage. Also the problem with unarned defenders is you should already have had weapons not that you are using them. I think that you should have a limit on how many turns it takes to research a tech ie if planetary invasion is going to take to long to research right now research something else untill you get better infastructure. At least everyone should want extreme planets, and advanced versions at some point. Planet trading, resource trading, survey ship trading nerds to be more difficult. I did like how you made It harder to treaty trade.
An exploit I loved from two Iwouldn't mind seeing again is where the ai is bragging about what it was researching. If you haven't fixed it and still trade off this trade from this. When I research I go for survey tech one, and build a fleet of cargo hull survey shipd from this. Morale techs are good for researching early. Military, tesrarch, economics, and population growth techs are good to start off with. Were not going to control approval this time around other wise I was going to recommend 100 percent for as long as possible. Kerp production as high financially possible without shutting down the computer will need to monitor this based on expenses vs income. If you are implementing my combined research idea, or the idea of suggesting techs this could be used to your advantage. You could make it harder to make treaties, and trade at higher levels. At higher levels don't build defensive, and influence structures unless on border planets at higher levels. You could colony rush harder at harder levels.
Considering the fact that I can run circles around everyonein making wonders It would be harder if at harder levels their were fewer trade goods, and galactic achievements, but more super projects evening out the playing field for everyone. Their will be a time when a tech will provide buildings with better bonuses, or the same bonus but cheaper. This is time to upgrade that one per planet building. At higher levels you can make it where bonuses from multiple versions of the same super project don't stack. You can make the ai more prone to be hostile towards you at higher levels. At higher levels you can make the ai less likely to vote with you on the galactic council, or vote in a way to make it really hard on you, and then withdraw from the galactic council. Make a level ehere it only builds economic structures.
Ai more proned to savatoge through espionage at harder levels. Ai more proned to allie against you with each other at harder levels. Pirates harder at harder levels. Ai more proned to allie with pirates at higher levels. More often negative random events at harder levels. More significant nrgative random events at harder levels. Some ais not researching weapons, or building warships is a problem. Ai less likely to send you freighters, and more prone to send freighters to each other at higher levels. More likely to trade emvbargo you at higher levels. Attempts more to steal techs from you at higher levels.
Altarians more proned to use more people againstbyouif attacked. If there Is a new super instigater more proned to brive people to attack you at higher levels. More ais are more likely to intimidate you, or declare war on you at higher levels. More ais build up quicker on higher levels