As for the story:
1. Play as Revan (but still with his new identity that you chose last game, just have it explain what has happened so far in the beginning.)
2. Jolee with him
3. Takes place during AND after Kotor 2. (sequals don't ALWAYS have to be directly after.)
4. Revan loses his arm in true, Star Wars tradition.
5. Revan have a face-to-face confrontation with the main villan where the arm slicing takes place. (kinda like the Anakin-Dooku storyline, where Anakin later comes back and whoops him.)
6. A party member gotta die just half way through so you know that nobodies safe, and later kill off some more (but Revan and Jolee HAVE to survive!)
As for the gameplay:
1. Animations have characters actually move (i mean walk and run around while fighting.)
2. Vehicles....please, let us use the Ebon Hawk!
3. Jumping/croutching. Revan is so dang powerful his force jump gotta be like flying!
4. More puzzles.
5. Cutscenes that don't use the game engine. We need some amazingly realistic looking stuff.
6. Continue after the end. Little quests and assignments (such as escort, delivery, defence, and stuff like that.) to let you keep playing. Just like Fable did.
Well, first Kotor3 has gotta be announced before all my wishes can come true :P