Just a quick little thought:

Do you think Tarkin should play a major role?

He was given command of the Empire's greatest weapon.

He could also order Vader about.

That must put him pretty close to the emperor.

Imagine this whole thing starting with him as a good guy too.

OR maybe on the opposing side of the conflict and the Emperor's window into controling both sides. The bad Guy in Episode 2, but surrenders and becomes and ally in Episode 3 when the tide turns against the Jedi.

hmmmmmm....