The main reason for doing the missions is to find information about the sith cult. The idea behind that is not bad but the problem is that the missions don't really have much to do with that (except for the hoth, vjun, taspir and korriban levels). Same thing with Rosh joining the dark side. It's a good idea to bring emotions into the game. Problem is that no one cared for Rosh in the first place. I think introducing completely new main characters might be too much. But having a couple more encounters with the existing enemies (Alora, Tavion, Rosh(?)) could definitely help the story.