BYU because first I’m a BYU homer, but second the only card the P12 has to play right now is to get bigger before everyone else does.
Go to 20 teams first, negotiate a gigantic TV deal and then go on defense. Own the Mountain and Pacific time zones and compete in the Central time zone.
You may still lose a couple schools over the years as the SEC poaches from the B10 and then they poach from the the new P20, but at 20 teams you can absorb some of that and just keep on chugging. Hope that Okie State or Texas Tech or BYU or Baylor develop into a really great program with the new resources (in the case of BYU) or without Texas and OU sucking the oxygen out of the room (in the case of OSU, TT, etc). Give USC a chance to play in Texas every year and compete for Texas recruits.
You could do 5x four-team pods in that conference to keep things mostly regional. BYU, ASU, AU, UU… CU, KSU, ISU, KU… OSU, TT, Baylor, TCU… but then have some regular and rotating cross divisional games.
It will never equal the money the SEC and B10 have but nothing is going to do that, and this will at least help keep you relevant. No one on ESPN will be able to talk about football in the Central, Mountain or Pacific time zones without taking about your conference. No P5 recruit will be able to play football in those time zones without doing so in your conference.