In short, I think you are thinking too much within the geographical and cultural confines of conferences, as evidenced by the "like they're going to leave the Big 12" comment. If it were a simple matter of a geographical/cultural addition, I'd agree that Pitt would be the best choice. This expansion talk, however, is all about money, especially in football. The Big Ten is looking into expanding into a new market, which is why you see Rutgers thrown in the mix - NYC sort of fits that bill.
The Pac 10 is pretty much all set to offer Colorado and Utah spots. The Big Ten wants to get in front of this rather than react, which is why the talk is settling around what they plan to do. The likely scenario emerging at this point is four super conferences, with the SEC ready to poach whatever the Big Ten leaves behind. The financial status of the Big 12 is not nearly as good as the Big Ten, which is why the Texas suggestion keeps coming up.
If expansion happens, it won't be by one team - it will either be by three teams or five teams. You won't see a 12-team Big Ten. This is not going to be an evolutionary change; it will be revolutionary.