They really should take a run at Texas A&M and Nebraska. It may be a longshot but i'm guessing both fanbases would rather be back in the Big 12 playing their old rivals. If the B12 can put together a real TV network that will come close to the money that the Big Ten and SEC networks are getting then maybe they could be persuaded.
The key is that both left because they felt Texas forced them out. Neither really wanted to leave.
Texas A&M especially may want to return. I'm sure they like the SEC but it's a tough conference and after the novelty of playing some new teams wears off, the fans may want to get back to playing the Texas teams.
From a football standpoint, Nebraska would almost certainly rather play Texas and Oklahoma than the Big Ten teams. Also, Nebraska will likely always struggle in the Big Ten. It's because they must recruit Texas to survive and get a few kids from Cali as well. That was their key to success.
The problem is Cali kids won't want to play almost all their games in the east and Texas kids want to play in Texas. Playing for teams like Oklahoma and Nebraska was fine because they still ended up playing a bunch in Texas anyway.
But the Big Ten is more than an athletic conference. The universities are linked academically. They will share research. Professors visit other schools. The Big Ten schools are all very good and they also have alliances with other schools like Johns Hopkins who plays in the Big Ten for Lacrosse.
The issue for Texas A&M might come down more to money but there is more involved for Nebraska. The question for Nebraska would be, do they want to be a great football team or a great university? Another few seasons of mediocrity and that answer may change.