Honestly, if you do the Luke side of things, maybe have him leave the planet with Biggs, who was planning to ally with the Rebels. Thus giving him the chance to be involved with events in a different way. Never rising to become a Jedi, but instead following a different path. 

Heck, you can even bring in Ferus Olin, who was Anakin/Vaders old school buddy and worked for the Empire at one point while trying to be a double agent. (Eventually Ferus dies fighting Vader between episode 4 and 5.)