Hmmm, well I have very mixed opinions about the whole thing. Not my type of game for the most part, and the reviews didn't have me running out to make an exception. My interest in the second one depends entirely on how well they improve it, any way it goes I'm not running out screaming to buy it.
Force Unleashed is one of those games I might pick up for $10 on a console, or download for a similar price (or less) on something like Steam. In both cases I'd have to be fairly bored for whatever reason.
As far as the storyline goes, from what I heard the expansions and such had him going off to kick Luke Skywalker's butt and everything else. It definatly ran into the whole "alternative universe" type thing.
It's definatly not the first "seperate universe" because you had a whole series of Star Wars sequels featuring the concept of the Moffs/Nobles of The Empire putting up a fake 'Son of The Emperor' to lead the Empire as their puppet. He was named Triclops or whatever since he had three eyes, and possesed no force powers at all but faked them with cybernetic implants (lightning blasters built into his hands, etc...) to sell his legitimacy to the majority of people who had no idea about The Force. (The Glove Of Darth Vader, etc...). Some concepts from that universe DID make it into the general canon in some form, though Timothy Zahn's directly contridictory future seems to have become the canon with pretty much everything from that series having become total "elseworlds" type fodder, despite it having some interesting concepts for the age level it was written for.
Really Starkiller is no worse than Triclops (I think I remember the name right) was. A character can be interesting without being canon. I've always kind of thought that bringing out Triclops as a hidden character in a Star Wars games just for the sheer WTF goofyness of it would be kind of cool. It would have a lot of people going 'WTF was that guy that just wiped me out' and probably looking into a version of the universe few people remember.
The big problem with the non-Zahn sequel was the age level it was written for. Some of it was far too kiddy to work. On a certain pragmatic level however I could always see the logic behind the Moffs trying to produce a puppet heir like that, rather than letthing Thrawn step up quite like he did. What's more I sort of suspect that if they did Thrawn might very well have fallen into line under him as he did seem relatively loyal before Papaltine's death. Written in a more serious fashion it could have been truely epic. I also thought the idea of using cyberware to fake the force powers was quite interesting especially when it was written. Things like lightning blasters, micro tractor beams, and other things... he was actually a pretty BA character.