Pretty much every strategy game i've ever played has terrible diplomacy. "Alliances" last a few turns and then your ally turns on you for no apparent reason. For the love of mustard, please do not let this game fall into that pit.
Kickass Single Player diplomacy would definitely set Sins apart and get it great reviews. I can't agree more.
Some gifts to other factions should offer *permenant* bonuses to your relations. That is, if you give a struggling faction a few systems, it should be almost unthinkable for them to attack you for the usual reasons the AI attacks you.
Actually, everything could offer permanent bonuses. If you ever want to change your net relations, just do good or bad things to another faction. As for giving them a bunch of systems, why, that would give you an enormous amount of positive relations. But I don't think they should ever forget about the time I gave them a couple credits.
All diplomacy options should have a specific, negotiable duration and should be unbreakable except for the case when two of your allies go to war with each other, then you have to choose which to support. If you dislike this feature, i would at least ask that whenever a player cancels a peace treaty that all their ships inside your territory be warped back to one of his colonies to prevent cheap shots ( Civ 4 style ).
I disagree. First, about warping ships away: If we add neutrality to the mix (right now all we have is friendly and hostile), allies can still issue warnings to keep your forces away from X planet or you'll pay. You should be able to do the same (As well as annoy them about building more tradeports, for example). You already get messages about where fleets are, so if you're worried about the position of a neutral fleet, yell at them, and if you're worried about the position of an allied fleet, you should have the option to break the peace treaty at this point.
Allow permanent alliances to be formed. THis should be hard to pull off, require a decent amount of time spent with good \ great relations and maybe require a few gifts. This would change the AI's stance toward you so that it would be identical to a game where you started off on the same team. That is, the AI should never again consider you a threat, and never conspire against you. It would probably be best if you could only make a permanent alliance with *

faction during a game.
As long as you get an allied victory, fine.
As you begin to dominate the game, the factions with a neutral, or hostile stance toward you should realize that you are the main threat to the universe and start to band together. They should pool their fleets and make a concentrated effort to destroy your ships and take back a few regions. If successful, they should revert back to their old hostilities and begin fighting again. THe same should happen if an AI faction is dominating, the other factions\human players should band together to try and stop him. If implemented, this will make the end game a lot more fun and make it very challenging to steam roll the whole universe.
It would be a challenge to implement, I'd think, but would make the game sooooo much better. Perhaps this could go hand and hand with your next idea.
Shared campaigns. When dealing with an ally or potential ally, you could offer a shared campaign against another faction. This option would bring up a map of the enemy's systems and the player \ AI would divide up those regions among themselves. IF an agreement is made, those systems automatically go to their rightful owner once conquered. Each side would have to pledge ships to take part, and the player would get to choose who gets control of those ships. THat is, if the AI offered some ships and you wanted to control them for the duration of the campaign, that would be possible. The opposite would be possible as well, that is, you could let the AI control your ships so you can focus on other things.
Another difficult but potentially amazing feature.
So, to summarize:
-Diplomacy in this game MUST be drop-dead amazing in order for it to stand out as far as it truly should. This is the difference between an 8 and a 9 in a review score.
-Everything you do to another faction has a permanent effect that is visible, Civ 4 style. To change your relations, you must make up for what you've done so far.
-Enable the ability to message AI players about keeping their forces away from other planets, building more tradeports, asking for aid, and perhaps even saying "Your destruction is inevitable" could have a (positive or negative) effect, making them either fear or resent you.
-Peace treaties should be breakable at any time for any reason, but the AI must have a good reason (and EXPLAIN WHY THEY'RE DOING IT, like Master of Orion 3 didn't do).
-Alliances, however, cannot be broken. You do get allied victory, though.
-Shared campaigns, orchestrated by you or the AI, are an amazing idea.