Easy one this Gizz,
Frontier.
I'd love to see a new version of Frontier with better AI, more ships and a more detailed universe to explore. The sheer open-endedness of the game made it playable in a way that few games since can equal. Better yet, a new version of Frontier would lend itself well to online gaming, as has been seen recently with Eve.
Speaking of which, does anyone here subscribe to Eve? From what I've seen of it I'd give it a go were it not for the fact there's no broadband in my neck of the woods.