I don't think we want to go to 12 teams, because wouldn't our schedule then become unbalanced? I don't think you're allowed to play 24 conference games.
Regarding Butler, I'd love to have them in the Valley, but I don't see why a team would leave the Horizon League, since I think they're almost as good as us. A lot of the "mid-majors" I'd want to bring into the Valley seem to be in the Horizon League already. (Butler, Loyola, UIC, UWM)
I think SLU would be nice... I'd also actually like to see DePaul in the MVC (since I think the Big Least will get a bit less big in the next few years-- they have waaaaaaaay too many teams). I also always think of Tulsa coming back. That being said, some of the "weaker" programs, like UE, Drake, and InSU have been doing pretty well this year, so I don't know that I'd chase any of them off.