The game is really great as is, so I don't think the first expansion should add a lot of fluff. I think it would be better spent expanding upon what we currently have. I would like to see some of the most popular features from 3rd edition return, however.
1. Give us more system tiles, public objectives, secret objectives, and one more racial tech per race. Additional variety would really help future proof the game. I'm not opposed to more actions and political cards, but the decks for those are really great as is.
2. Give us the 7 - 8 player option. This is easy as it only requires more plastic and more systems from above.
3. Give us some form of Distant Suns. I would really love to see this return to a 4X game and give players more random elements on the board to adapt to.
4. Add in Leaders (this just adds some additonal flavor and complexity for those that want it)
The following are things I always found cool, but fiddly and unneeded. They would be perfect for a second expansion that targets people that want more variety:
Mechs and Shock Troops
Politicians
Space Mines
Colonies
Mercenaries