I'm just going to vomit my opinion on you guys, since this is one of the few realignment threads out there and the BC boards hate these kinds of things:
1) I think 16 is the magic number. Can't see 18 or 20 team conferences. Sure, you get more markets, but there's no way to make anything higher than 16 work. With 20 teams and a 9 game schedule, you never play a team from the other division unless you make the championship game. Never. Even with 18 teams and a 9 game schedule, you only play the teams from the other division once a decade. Teams are already upset about the infrequency of match-ups in the 14-team ACC, BIG and SEC.
2) Notre Dame will join a conference if 4x16 is happening, and the BIG, Big 12 and ACC won't go to 16 without fully exhausting all opportunities to land the biggest chip remaining (ND).
3) ND's preference is probably BIG > ACC > Big 12.
4) If ND goes to the BIG as #15, then things get really interesting for #16. I just can't see any of the other commonly cited BIG target teams leaving alone with ND without a rival partner. UNC, Duke or GT are not going alone to the BIG without a more historical partner. Maybe VT would go alone. That might be the most likely partner.
5) However, if ND requested BC as a BIG partner (assuming the pope intercedes), BC would be approved in a heartbeat by the BIG. After all, BC brings Boston the same way Rutgers brings NYC (in that they both could potentially bring in viewers and command top rate carriage fees with improvement). BC with historically respectable basketball and football (though we've picked a terrible time to struggle), and the best hockey team in the country, which would certainly win some votes with schools like Wisconsin and Minnesota. Also, if you're ignoring AAU to add Notre Dame, BC's academics certainly pass muster.
6) Rule #1 of Conference Reallignment. (am I doing this right?)
