Cincinnati seems like a no-brainer, and BYU seems like the more logical choice than Boise State, at least to me. It's not like the Big 12 is anti-religious schools like the Pac-12 is. SLC is a substantially bigger TV market than Boise (even if you split it with Utah), and BYU has a much larger national brand that is not going anywhere once its football team returns to meh. Their success at the national level is also much longer term than Boise State, who has really only been notable for the last 10 years or so. The academics are Boise State's biggest obstacle; they were a glorified community college 15 years ago, and have a ways to go before catching up, even with the lowest academic schools in any of the Power 5 conferences. I don't know the exact numbers there, but I know this came up a few years ago.
BYU's problems seem tied to its religious affiliation. They won't play on Sundays, which wouldn't be much of a problem for football, but might be for other sports (basketball tournament?). I also don't know how TCU and Baylor are going to feel about the LDS church. It might be worse to have competing sects than no religion at all.
The question to me is do they move Texas & Oklahoma into different divisions? A zipper with protected rivals makes more sense, as geographically it would be just as unbalanced as it was before the exodus if they did north-south. Maybe the "south" division could include all the Texas teams and West Virginia and Cincy? That would leave the north with the 4 Oklahoma and Kansas schools, but Iowa State and BYU/Boise. Oklahoma and Texas could have a protected rivalry so they still play every year, and might play twice. I don't know if there are any other extra-divisional rivalries that they need to protect, but I know the Big 10 only has one protected (Purdue-Indiana), so I would think the Big 12 could make it work as well.
Or equally likely, they'll petition the NCAA to get a title game with 10 teams. I know the NCAA shot that down before when the Pac-10 tried to do it, same with the Big 10 when it had 11 teams, but the post-season structure is different now, so maybe it will work this time.
XI Wiki



