Not sure what I'd like to see with Diplomacy, as that is a more 4X type of feature typically. Best part of Sins is the way it combines RTS and 4X for a faster paced game (although you can still have long games) while maintaining the best parts of 4X play.
However, the one thing that needs an overhaul is Culture. It's useless right now. Advent can sometimes take over a planet with culture and cuase it to go neutral, but most others end up just seeing the culture top out and even out/allegience doesnt drop. Also, the bonuses for culture are quite weak in regards to how much you need to spend to spread it and not even in the slightest. For the itme and effort put into Culture, you could have just put the money into ships and dominated the opponent in half the time.
One option is to combine the various levels of culture upgrades to make it easier to get access to. Another is increase the rate of culture spred. The best option is to combine those options and add later level upgrades that would flesh out culture and allow you to convert populations without a fight. People should be in awe of your culture and oust their leaders in favour of joining your culture/empire. Simply making the planet go neutral doesn't do anything for this.
How to goa bout this is a question for you to answer. Maybe spies and counter spy agendas (some could use terrorist tactics while others peace rallies and advent could be psychic brainwashing or some such). The problem arises when you make this too complex / deep and the game loses the appeal of the RTS/4X hybrid.
Computer AI needs an overhaul. I do most of my Sins offline against CPUs. Easy and Normal is pathetic, but a good intro for new people. Hard is barely a challenge and Unfair, save for small map Aggressors, are little challenge either. Early game is the only time they really do anything. Maybe add an insanity option for AIs for the rest of us or just make them a little smarter. Ive played Unfair CPUs that went back and forth between planets (3 phase jumps) as I assualted from both sides and ran away as soon as he jumped into the planet and he just eventually quit.
Speaking of which, CPU surrenders are good ideas, but are quite annoying as well. In large games, Ive had a CPU quit with 5 or more planets all with fully upgraded defenses, mines and star bases and no culture problems just because I killed their fleet that attacked my colony. Not a big deal, but then I have to clear out these huge defended bases that take forever to take out just to get to the bases despite him being "defeated". Defenses should shut down and be salvagable by scouts / colony ships or claimed when I take the planet. I shouldnt have to wade through them.
Also, it makes it difficult to get the achievements for Civiv and Military-only research when the cpu quits so easily in 1v1. Maybe an option to disable full surrender or let you "accept" their terms of surrender or deny them in favour of full out destruction.
To go with Culture, we could have a galactic peace or council or some kind of universal senate like other 4X games have. Not sure if it's worth for multiplayer (depends on how its executed, but people wont vote for you/make diplomatic deals that would get you elected/victory), but could be a great single player option.
Things to think of are number of treaties you have with other races (trade, shared vision, cease fires, etc worth X # of points), how far your culture has spred, # of artifacts (thats kind of random though), "diplomatic research" (if you add a new tree with diplomacy research only), maybe build "wonders" like in other games on your homeworld that take forever/get announced to enemies like artifacts/increases culture 10 fold or something, and various other criteria that add up to you eventually establishing a galactic council/declaring peace(aka victory for you) and so on.
Im not sure how to execute this, but you shouldn't have to fight a single enemy to win this game if you dont want to. Currenlty, that's impossible to do outside of rare culture wins on small maps that saw you dominate the computer early anyways.
Missions are terrible right now. It's 5 minutes in and they demand 1000 metal or later on want me to destroy a civilization in anothe rstar system or what have you. I typically just ignore them as there is absolutely no gain from it for me. Teaming up with the computer doesn't help me in the least, even when I ping the map to ask him to help attack or defend, and I just end up playing solo even on a teamup. Small trade boost is all I get out of it.
These missions should be a little "easier" and related to what I'm currently doing. Some peace missions would be a nice touch as well. I shouldnt be forced to go wage war on some other culture just because dude wants me to. Add tariffs to my trade income if I dont complete a mission or forced compensation. Add some diplomatic envoys that are visible to all while moving and add incentives to my enemies for "assassinating" these people as they head to my allies homeworld from my own. If he makes it there fine, I get bonuses, if he dies, our relations sour significantly and enemy that killed the diplomatic envoy gets huge boost with my current ally.
Just give me a reasont o do these missions and dont force them down my throat with ridiculous demands. Allow me to give a mission as well.
Would like to see new Achivements added to the game. Ive just about finished all of them (cant seem to get a computer to stick around while Ir esearch every civic or military only achievements though) and would like to see a few new ones. Could add in new ones to go with the Entrenchment expac at the same time (maybe build star base in hostile gravity well, fully upgrade a star base, destroy an enemy star base, etc) to gow ith any new Diplomacy ones you add in (some kind of culture win, voted to galactic council, etc for whatever you add). Just adds some fun stuff to attempt to do with the game that aren't exactly complex to implement either.
Fix some of the capital ships. Several are quite overpowered in comparison to others. The few that are 'support' Capital ships are quite weak due to the dependancy on Anti-Matter and don't even make up for it in late game compared to a Marza or Kol, who both dominate early and late game. Other races are similar in that regard.
Late Game Turtles
In huge maps, it's quite difficult to progress in the latter parts of games. I'm talking multiple star systems or 50+ planet maps where I can expand and conquer a great deal of territory early, but am forced to then consolidate defenses or lose everything. Computer does the same thing though and then proceeds to take it even further than I do (comp usually lags in research in favour of turtles) with mines everywhere and upgarded starbases on every planet. It's just a really hard nut to crack that has me stop and start for 3 or 4 hours to finish the last half of the map / expand beyond the solar system I jus tconquered to the other 2-3.
Yes, there are siege ships and planet bombers and so on, but when there's 5 hangar bays and another 6 or so from the star base and 8 or 9 turrets, a gravity well filled with mines and the enemies army there, it's hard to get through the first time and as soon as you finish, you have to secure the base, rebuild your army and do it again at the next one and the next one and so on, even if the computer "surrenders" (see above).
If I play on a small map, resources are tighter, fleets are smaller and star bases less abundant and I can finish games in an hour or two at most. Anything above one solar system/30-40 planets and it goes to hell for a 6 hour+ marathon of defenses / fleet cracking. Cultural victories / conversions could go a long way to helping this in longer games. Surrenders could be full on joining my army types of moves too that would stop me from having to destroy their worlds and defenses while allowing me to still expand.
Refineries are almost useless when compared to trade, especially as the game progresses. Early on,t he trade port is far cheaper than the 1300+ resource refinery, quicker to research and gives me mroe income at the same time that will continue to grow as the game goes longer. Refineries give a small % boost to resource income that is already quite small and shrinks drastically as the game goes on with fleet upkeep. Ive had games with trade easily exceeding my planetary tax incomes while refineries are capped at 3 per node and take huge hits the longer the game goes on (Ive been raking in 100'sof credits per second before fleet tax while resources are 10 or something before tax). Refineries are too expensive to set up for such small gains too.
I like the way trade works right now, but it's just far too dominating with how weak culture and refineries are. Just rake in cash, buy resources and destroy the enemy is far easier than low resource gains or trying to take over a planet with culture, making mass refineries or temples bad options. Trade routes could use a work though. Maybe let us designate the trade routes for maximum effect (no duplicate nodes/completing of "cycles" though). I hate having a long chain broken up because I placed a trade route somewhere that caused it to be changed to a smaller chain.
Finally, tax from planets is quite weak. Auxillary governmetns from star bases are useless in most cases too, so maybe make that something that increases tax rate by 1.0 and 2.0 or some such to offset the weak tax on planets while stills ecuring the planet from being capped by enemies. This wouldn't be the 14-20range income of a homeworld, but would allow you to still up the income on high population planets to ~8-12.0 range for nice gains. This could also be a diplomatic research logistics item / planetary upgrade that adds an embassy to the planet for more tax.
Some options to fix these would be to have refineries be flat, non taxable/fleet size affected structures (keep the node limits of 0/3 up to 3/3 though to prevent abuse). This would make building one refinery every other planet quite an enticing option, but could use some work. Might just have to up rate of income instead of removing tax, as I dont havethe numbers on handto make / claim a change like that wouldnt be broken or not. Fixing culture like I mention above (or at least looking at some form of cultural benefit/win beyond my options) could make building temples a better option than the 3-5 trade ports perplanet that evenw ith full fleet penalties saw me making 200+ credits per second in some larger maps and never having to worry about minerals.