Starkiller is one of Star Wars' most action figure-y characters, thanks to his debuting in a video game at the height of the franchise's 'anything goes' approach. He single-handedly takes down a Star Destroyer with the force, cuts down countless Jedi with ease, and grows to a power level that few in the movies and shows rival.
It obviously wasn't canon at the time, but it was a hit, and Starkiller's popularity even saw his character expanded in novels. The games try to fit into the wider universe now, so the chances of seeing Starkiller again are even slimmer, but fans are hoping he pops up as the villain in the third Jedi game nonetheless.
"Who do you want to be the main villain in the next [Jedi] game?" MainlyPardoo asked on the official subreddit. "This is my pick," they said, attaching a photo of Starkiller dressed up as an inquisitor.
The Force Unleashed came out in 2008 before Disney bought LucasFilm, so the game predates the idea of inquisitors. These are the Empire's evil force users who hunt Jedi that survived Order 66. They aren't technically Sith, bypassing the whole 'only two can coexist' rule, whereas Starkiller at the time was a hidden apprentice, like Asajj Ventress and Count Dooku.
However, not everyone is completely on board with the idea. Vision_dynamic replied, "Unfortunately, Starkiller would destroy Cal or need to be nerfed into oblivion and would only be Starkiller in name. Would be cool as hell though. Maybe make Cal stronger over the gap between this and the next game and have Kata with Cal the whole time as a companion to double team Starkiller? Or Kata be as powerful as a cheaper out?"
Others argue that Starkiller can't fit into the current canon because of his power, period. Game or not. As I said, he's the definition of a Star Wars action figure, being an immensely powerful force user. His entire purpose was to let Star Wars fans live out a power fantasy that went above and beyond anything else we'd seen up to that point. So, good luck explaining how he died off-screen or why he didn't help at all in any of the films.
Another option floated in the comments is that Darth Vader returns as a villain, this time in a fight. Obviously, Cal can't mortally wound him, but it'd show how far he has come since the first game where he had to run away. Whatever choice Respawn makes, it'll be a few years yet before the trilogy comes to a close.
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