instead of a full out SDK, there should at least be a mission designer for Deus Ex. Like a flight sim's mission designer, here you wouldn't be able to modify the content or levels directly, instead you would only be able to change the objectives, enemies, storyline, etc. So basically you'd be playing the same maps over and over, but since Deus Ex's world is so large, with hundreds of rooms, drawers, and computers to access.. the replay value would still be huge. Custom mission designers could create new experiences in the original environments.
I wish all FPS games would attempt to do this. So far not a single FPS game has. Instead they release a really complicated set of editor tools that less than 1% of its fanbase ever figure out how to use. With dedication and countless hours of work, a die-hard fan can create a brilliant new map. But they are few and far between. Some day, I hope, mission designers will be catered to the casual gamer... and let people just enjoy the preset maps in new and simple ways. (Think Warcraft/Starcraft map editors)
Oh, and adding multiplayer co-op would be cool too.