Starkiller takes Vader's place, and probably becomes military Executor. Like Vader before him, he has no chance of defeating the Emperor on his own (not to say he did in the first place), so he has to run around doing Sidious' dirty work.
However, the rebellion wasn't formed, and might never be formed in the first place. Come 0 BBY, the Death Star is finished, but there isn't a rebellion to steal its plans. The galaxy's Imperialization process would have undoubtedly gone much smoother without any organized rebel groups.
If there is no rebellion to steal the Death Star plans, the droids never arrive on Tatooine, which stands a good chance of postponing old Ben Kenobi's meeting with Luke. If Starkiller ever discovers Luke, he'll undoubtedly try to use him as an apprentice, but since he isn't Luke's father, it might never work.
One way or another, it would probably culminate in a ROTJ-esque confrontation with Starkiller replacing Vader, and probably no organized rebellion. If Luke turns, Starkiller dies. If Luke doesn't beat Starkiller and doesn't turn, he dies. If Luke beats Starkiller and then refuses to turn, he'll die. Starkiller isn't Luke's father, so he's not likely to care whether Palpatine fries him or not, because if Luke won't turn, he's useless to them both.
If not, and Starkiller somehow manages to train Luke as a secret apprentice, then Palpatine kills Starkiller after discovering them, because he knows better than to not do so.
Not sure what happens to Leia, though. She'd probably be dead two years from a stormtrooper firing squad or something by the time the Death Star's completed.
"Grant Allen [...] had written a book about the Evolution of the Idea of God. [...] it would be much more interesting if God wrote a book about the evolution of the idea of Grant Allen." ~ G. K. Chesterton, The Everlasting Man
Last edited by TKA-001; 01-25-2009 at 04:28 PM.