A rebuilt Homeworld: Cataclysm. Unfortunately, most of the assets and source were lost, so they would have to be re-created from scratch.
NOLF, as has been mentioned.
The X-Wing and TIE Fighter games would be amazing.
I would absolutely LOVE to see a new Starfleet Command game with a modern engine, good writing and the depth of SFC II: Empires at War. Hell, I wouldn't even care if it were JJTrek as long as it had good writing and the same depth as SFCII:EAW. SFCIII was crap in every way.
A remake of The 7th Guest and 11th Hour with higher resolution assets designed for 1080p/4k, and in the case of 11th Hour better puzzle balancing. That mousetrap/maze puzzle is a bitch on newer machines!
A new Freespace game would be great - 2 ended on a cliffhanger.