Shape-shifting + reactions...
I think a fun mod would be the capability to shape-shift into any character model whenever you want, and to receive reactions from npc's.--Like on taris in the first game, when you wear the sith armor. EVERYONE reacts to you now as though you are a sith. That would be cool, especially if you could change the plot by shape shifting in particular situations. I know it sounds hard, or a lot of work, But I believe it would be QUITE worth it.