Nancy, we'd never guess that you liked Michael Fassbender,
I think it is going to totally depend on how the writers write for the new Batman.
I loved Michael Keaton in the original series of Batman, I thought he did an excellent job in that role. His personality fit with the lighter hearted superhero (not so much Clooney and Kilmer, though).
Then the Dark Knight series came out and Christian Bale was perfect for the darker and brooding Batman. Keaton couldn't have pulled that off, and Bale wouldn't have been good as the earlier Batman.
I don't know how they are planning to have this one act - somewhere in between? I can't see Affleck doing either one of the other roles - he has his own acting style.