DominicCobb said:

...I can't see a single reason why she should be the villain, other than it would give her something interesting to do.

There has to be something interesting happening with Leia or there is no point in making her "the other", Luke' sister, Annikin's daughter, ect. That plot thread was started in ESB, expanded upon in ROTJ, shown in ROTS, but never came to anything. In my mind, it's one of the only reasons for a sequel to ROTJ to exist. It's the only loose end when the credits roll on Jedi.