Playing Devil's Advocate here, there [i]is[/i] a universe (The Next Avengers movie AU) where Ultron wiped the floor with the Avengers. Problem is that years later, he got his ass kicked by the Hulk. In a movie that's supposed to be about the next generation of Avengers.

That said, I think what Clanjos is saying is that we have to use canon backstories and not hypothetical what ifs. Why he's here is already answered for in the RPG itself. Wish-granting entity summons people to help cure him of something terminal and in return he can grant them their greatest wish via messing up with probabilities. For example, Ultron's wish might be to kill the Avengers and the probabilities of Ultron losing to the Avengers get skewered while odds of him killing them get increased to "it was meant to be" levels.