First of all, I would hope any fix type things go to all the people who already have GalCiv for free. I've been very insulted in the past with companies who tried to sell "fixes" as an added upgrade.
As for new features that might get my attention as someone who is slowly losing interest in GalCiv.
More ships, maybe option to customize ships, new races. I think you see a trend. If I'm going to shell out more money, I want NEW features, not just fixing what is already there.
Although, this may sound funny, I think fixing what is there now is more important. There are certain things that make playability very difficult, especially in late games in large universes. Other, more frequent posters to the forums have already expressed these ideas, so I won't bother doing it again, except one.
Waypoints are a great addition, but the ability to move a waypoint would probably be not that difficult, but would be a massive improvement to late game playability in large games. It's the little things like this, that while not terribly sexy, or exciting to code, that would make GalCiv, in it's present form, a much better game.
Although, let's also give credit where it's due. The free expansion pack is great, and is very much appreciated! It got me back playing, for awhile at least.
I'm going away on a business trip this week, and GalCiv is one of those games that still plays nicely on the laptop, so I'll be submitting a few more scores hopefully.