Just for grins, and to answer the OP question, I would say AAC.
You get more TV coverage in the earlier time zone, and more ESPN love. The Pac12 just about gets looked over, the MWC would be worse. I think the travel in the AAC would be easier and cheaper too. BBall is better in AAC by a lot, and imho the football is crap in both.
I also think Surly is onto something in that it wouldn't be just ISU - it would probably be 3 or 5 other teams as well "left behind" and we'd pull others (ie most of the AAC) into the Big12 since the brand is better.
Let's get super speculative and creative. Say KU, KSU, Baylor, TCU, ISU and WV are "left behind". You could go for the gusto of 12 adds to make a super conference with 3 divisions east/west/central.
EAST
UCF
USF
Temple
UConn
East Carolina
Navy
MIDWEST
ISU
Memphis
Cincinnati
KU
KST
WV
SOUTHWEST
TCU
Baylor
SMU
Houston
Tulane
Tulsa
That midwest division is a HELL of a bball league. The SW football isn't bad either.
For FB you'd play the 5 in your division, and 2 from each other every year for a 9 game schedule. So the other division teams you would see every third year, and every 6th at home. Not ideal, but such is life in the mega conference.
For BB I am struggling if you would play a 17 game season and just play everybody once, or add 5 games, play 22 conference games, and play home-and-home in your division. That's a LOT of conference games. Maybe you play h&h in division, and just 3 of each other division for 16 conference games.
And yes, this topic is not immediate, the Big12 isn't collapsing. And I guarantee the above BS is never going to happen. But why not BS with it? It is the offseason, why not talk about something? People carping about the off season topics not being good enough need to get bent. Or go on facebook and talk about politics, or watch cat videos, or something.
Conference realignment is the cat videos of CF.