Needs:
1) More portal stuf - LOTS more. I want some downright dizzying mindgames with portals this time, not just the simple doorways & spawn points.
2) More varied environments. I understand the game took place in a ship, so there was only so much that could have been done. Still, it started to feel like Doom3 felt after a while with the same thing over and over.
3) As said, the weapon sounds need more kick to them.
4) A longer game.
5) It seems like all the interactivity was prior to the abduction. I don't need to tinker with EVERYTHING, but let's spread it out a bit.
6) Multiplayer bots. I realize with the portals & all that this wouldn't be easy, but MP games are useless to me without bots (no high speed available in my area).
Doesn't need:
1) Vista exclusivity. Not unless we're talking 2008-2009 release.
2) Spirit walk hints. As auto-difficulty adjusts, these need to disappear for more skilled players. (Or better yet, how about false ones?

)
I hope none of these suggestions are taken by Human Head as gripes, as they're certainly not meant to be. I loved Prey!