1. (NEW SHIPS FOR ONE OF THE FACTIONS)add unique ship types so the factions will not look totally the same, one of the 2 factions could inherit the original ones, maybe the loyalist ones, and rebels shoudl get new ship types not to look totally the same. Though I am aware of the 2GB RAM limit stuff or something and know that this cannot be done, so ignore it.
2. INCREASING RACIAL AND FACTIONAL DIFFERENCEs making races and factions more different from each other. This cannot be a problem, and will not make the game more energy consuming I guess, and the factions really need more specific stuff, not just a few that are actually used in games. I ignore those techs that are not doing anything useful like the Tec Rebel thing liberation at level8, we need more that are actually changing the gameplay of the faction. Maybe changing base hull-shield-armor ratios by a small amount would be awesome too. Maybe make Advent more crystal dependent but give them better crystal extraction rates, and a small bonus at the beginning (like in the E4X mod), or just make their crystal-increase techs easy to get, not 3-4-6, instead 1-2-4 or something.
3. DIPLOMACY (would be the best theme for the DLC). Total remake is needed, the way it is now, it is less useful than it should be (for me it seems totally useless), and factions (loyalist VS rebel) make allying even harder, and the whole diplomacy tech tree is too expensive and it takes too much time to get the pacts that will not really help you at all if you get them, because you need mutual relations that is a really bad idea. Good luck finding an AI that will actually send ships at your way, or a human ally, who is most of the times at war (latter is about online games) and will not have money for envoys, the whole thing is broken, so a nice expansion improving it would be the best I think, giving another layer of fun for the game (economy+military+diplomacy, but nowadays diplomacy is only found in noob ffa games I don't like playing, or AI ffa that is even worse). And mutual relations, wtf, I made him like me absolutely, but I don't like him enough because he has no envoys my planets so I can't offer him pacts, though I want to. It is a crap thing because I do want to give him that pact, though I can't because I dont like him enough, so, what?. .... paradox
4. REALISTIC MAPS (or just actually fun premade maps, not small like hell maps) maybe the map layout should be improved to be more realistic, I don't really like those solar systems that have 30 planets in them (or even more) (nowhere close to reality). In reality at best ~10-12 planet is the max for one system, not counting moons. In this game stars have a strange role, for me ruining the games where there are multiple systems because of the game structure, not really good to jump from star to star (like you reach the neighbouring star first, and not the outer Oort cloud or something..), so more realistic maps would be good, and premade maps that are actually fun, not those tight maps that will only let you get 2-4 planets before running out of neutral worlds and immediate conflict and etc. It woud be nice to have more maps that actually let you expand without running into the AI within 5 minutes of the game. Other maps beside the random ones are not much playable for me, I don't really like immediate conflict and I prefer to be at peace for like 5-10 minutes and just expanding.
Look at Endless Space, the galaxy view shows stars, and you can move between stars, and star systems have reasonable number of planets (1-6), and you have to click on the star to access planets and colonize, upgrade etc them. You owb the star system, attack star systems at a whole, in SoaSE the focus is totally on planets, totally ignoring star systems.
I have no idea how to make it better though, just it would be nice to see Sins be a bit more realistic in this field.
I guess we would need a new game to implement better and more realistic maps, though I think something could be done to make it better even in a small DLC. Maybe make random maps have some rules, so volcans will be near the star, ice will be further, making moon-like objects so dead ends from a gas giant or something. But I don't think devs want to deal with improving this.