I bought Sins of a Solar Empire pretty much immediately at release.  My initial impression was that it was very pretty, but there was no "story" to speak of, no campaign mode and that the AI was pretty irritating. 

After playing it for a while, I got bored with it and stopped.  At this point, I don't really have any plans to buy another Sins game.

I think Sins is very close to being a lot of fun, but ultimately isn't much fun, at least for me.  Here are my suggestions regarding what I'd like to see in a sequel.

1.  A campaign mode / some kind of story.  Honestly, the story in the rule book is not that interesting...and that story for all intents and purposes isn't really in the game at all.  I really think you should consider completely abandoning the story you've written for something more immediate and invoving which talk about specific characters and specific events.  It seems like it would have been really easy for you to make a series of custom maps which could be your "campaign mode" with opening and closing text blurbs giving you some kind of indication of story progression.  Why that's not in the game is beyond me.  It seems like the game was designed soely as a multiplayer experience, which IMO was a huge mistake.  All of the most successful RTS games have great story modes (WC, SC, C&C, Dune, etc)

2.  More time control options for single player, including slomo gradations all the way to stop.  Your game is gorgeous, however I feel like in battle I have no time to enjoy it, I'm too busy with micro.

3.  Punish hit and run bull**** more.  I cannot tell you how annoying it is to have "bad guys" constantly popping in and out of my space.  The jump engines on all large ships should be far slower.  If you get your capital ships caught by a larger force, you should probably die.  In the naval battles of WWII, (which this game kind of simulates) a damaged enemy couldn't just easily sail away from his problems by getting to the edge of an area then teleporting away.  Making comat more dangerous would help this game a lot.

4.  More custom formations/control of formations.  Unfortunately, most of the time, your ships tend to organize themselves in a big fat clump, not anything resembling a tightly organized military fleet.  If Homeworld could create fixed formations, what, 10 years ago, so can you. 

5.  The game should also reward in-combat tactics more.  The "anti-focus fire" modifier on shields really annoys me - why should I be punished for spotting the most vital enemy unit and killing it?  If you guys want to make certain units live longer, give them higher HP.  Ramming should be an option.  Damaged ships should function significantly worse than undamaged ships.  Combat should overall be significantly slower, but more meaningful when it does happen.

6.  Some kind of relationship between your game and actual stellar physics.  Your planets don't move.  Your stars don't move.  Your stars aren't "hot."  Your stars and your planets are clearly not to scale.  The first simple fix that I think you have to make is taking out the stars, or making them way, way bigger.  It's just ridiculous to find that the star your planet orbits is smaller than the earth-like planet you come from.  If you decide to go more ambitious, I think it wouldn't be that difficult to actually lay out realisitcly sized gravity wells, gas giants, earth-like planets and stars.  It'd just require a bit more work with your map editor and not use a standard size gravity well for every planet.

A third option would be to make the "big menu" more like an interstellar chart and less like something that's supposed to represent the way the map actually looks - so that when you go to a specific planet in the game, you can see that it has its own stars, gas giants, etc., they just aren't relevant for the purposes of the game so you can't send your ships there.

7.  Make the factions more different.  I know that this comment automatically invites a hundred "the factions are really different what about X or Y or Z".  I know that they're superficially different, however every faction has access to roughly the same early ships, roughly the same mid-game ships, roughly the same technology and roughly the same capital ships.  Yes, the very best players will find and make the most of the differences between the factions, however to a casual player like myself, after beating the game a large # of times with a single faction, I felt that switching to a "new" faction wasn't different enough to make me interested in playing the same game even more.  Of course, a campaign mode could help here too.

I really like a lot of aspects of this game, but I don't think it lived up to its potential, and I would have a hard time wanting to buy a sequel at this point.  I'd love to see some of the above issues addressed.