While I know that the entire genre is pretty much dead/buttoned up in it's entirety by EVE... But I'd love to see a sequel for FreeLancer and possibly Tachyon:The Fringe (possibly a remake).
Tachyon came out way back in 2000, but still looks alright (the gods only know how good it would've looked with modern graphics) but was really, really well put together gameplay wise, and for the time felt pretty big (before FreeLancer and EVE came along to redefine that word), but most importantly the characters felt real and certainly the Bora campaign was exquisitely well written. I don't include the GalSpan campaign in that because it didn't feel like a cause worth fighting for to be honest and didn't include those moments of corporate injustice that made my blood boil on several occasions in the Bora campaign but even so... It had it's flaws (like not being able to take off without a mission and so be unable to really take advantage of some of the features of the universe, and those missions being quite tightly scripted but... meh), but was still a really great game. Sequel would've been kinda nice, although possibly would have been a bit contrived since the campaigns tied everything off quite nicely.
FreeLancer is more recent by about three years, and really annoying since it left us a cliffhanger type ending, and massive potential and scope for a truly monumental sequel. A real shame since, while not quite as well written or acted, it was a phenomenal game, from pretty much every standpoint, design, gameplay and concepts. Sadly, FreeLancer 2 got nobbled when Digital Anvil was dissolved by Microsoft and so nothing ever came of the hypergates or the anvils that got dropped regarding further potential in the story.
But then again, space sims seem to be well and truly dead in anything other than the multiplayer market so....
Atleast Relic are hopefully not just coyly dragging us along for the ride on the fourth but unnamed project they're working on and we'll be getting a Homeworld 3.