After I posted my thoughts, I realized that
is just as likely to be used in Thor 2 given the way that The Avengers ends as he is in Avengers 2, and if he were involved in Thor 2, it would lead organically into Thor returning for Avengers 2.
I don't necessarily think they should
write the 'Big Three' out of Avengers 2, but thought I'd operate off of the potential for them to be absent that was written into the end of The Avengers and try to think about characters who could organically come in to fill their respective roles if Marvel did decide to not bring them back.
Even with an Avengers 2 cast lineup that includes IM, Cap, and Thor, though, my suggestion for Vision still stands, because it's an organic and suitable follow-up to what happens in The Avengers.