Re: How about a solo Avengers franchise?
Ha, you're right, Kang, he was introed in Avengers. Forgot about that.
I think, like Pym, if Ultron is the villain, Vision's origin fits right in, because that's the way the character was designed, to be able to fight the Avengers and then join them, same role Hawkeye had in the Avengers film really. And to take Chewy's thought another step: anybody can make an robot. Stark, Fury... they pull Vision out of SHIELD's Phase 2 cache if they wanted.
I don't think there are many characters like that though, who are built from the ground up in a way that a host of Avengers storylines can themselves function as origin stories for the characters.