Outside of Notre Dame, which won't happen, here's my list if I'm Jim Delaney:
1) Missouri - Would add a decent football program and basketball program. Already a rivalry school with Illinois. Adds a state to the conference.
2) Rutgers - Geographically, it's a leap, but to get close to NYC, I think they'd do it. Rutgers adds less than nothing on the bball side though.
3) Pittsburgh - Would be a reasonable addition in football (although, Wannstedt is a joke) and a great addition in bball. Would be a decent in-state rival for PSU as well.
If Mizzou turns out to be the choice, I think the Big 12 adds TCU. The only hiccups there is that the conferences would need to be re-balanced and there'd be 5 teams from Texas. Outside of TCU, I don't really know of a logical addition. Arkansas isn't leaving the SEC and non of the MWC or WAC teams outside of TCU would be good additions imho.