Which Big 12 schools? One thing I like about the ACC is that it still has its geographical integrity. I think at some point, when things begin to stabilize, that's going to matter - and the potential to control most/all of the East Coast media markets could matter too. If the ACC landed a Big 12 school, they'd probably always have one eye toward the exit just because it wouldn't be much fun to make that many trips to play on the East Coast and there would always be a more attractive destination.
I would only truly be interested in: Notre Dame, UConn, Rutgers, Temple and Penn State. If you get Notre Dame as a full member, you could consider Cinci or Louisville, but I think if you get ND it may mean you already have PSU, so you're set at that point with 16.
If you lose Maryland, I think you grab up one of the others on that list and make a hard play for PSU. Losing Maryland, media wise, is not a big deal. In football, Va Tech delivers the DC/Baltimore area as much, if not a lot more, than Maryland does anyway.