...then what happens to the rest of the Big 12 (which currently has only 10 institutions)? And which schools are in line to join the SEC along with Texas A&M?
Let's start with something glaring and obvious -- every sentence from this point forward is based on guesswork and predicated on the assumption that Texas A&M's leaders are indeed considering joining the SEC.
If Texas A&M joins the SEC, then the Big 12 conference is dead. Stick a fork in it, it's done. There is no way that conference can sustain itself with nine schools, especially recognizing that one of its elite members has said "see ya, y'all."
The Big East Conference also is raided and left to die.
The demise of those two conferences sets up the potential for four super conferences to be formed. I repeat -- potential. There is no guarantee that a radical realignment process takes place. But if it were to happen, four 16-team conferences would be a logical (though not only) outcome.
A 16-team Southeastern Conference. The current institutions would be joined by newcomers Texas A&M, Louisville, Cincinnati and South Florida. (In other words, one addition from the current Big 12 and three from the current Big East.)
A Pacific-16 Conference. The current 12 institutions would be joined by Texas, Oklahoma, Oklahoma State and Texas Tech. (In other words, four additions from the Big 12.)
A Big-10 conference that also swells to 16 teams. The current 12 institutions welcome Kansas, Kansas State, Missouri and Syracuse. (In other words, three newcomers from the Big 12 and one from the Big East.)
A 16-team Atlantic Coast Conference. The current 12 institutions welcome West Virginia, Pittsburgh, Connecticut and Rutgers, all four current members of the Big East.
Eight of the current nine football-playing schools in the Big East are picked up by super conferences, and it wouldn't come as a surprise if that ninth school -- Texas Christian -- somehow winds up in a super conference. But I don't see that happening. (More on the Horned Frogs below.) That leaves the league as a still-viable basketball conference with
and Notre Dame. But not Texas Christian, which I've not forgotten about.
Eight of the Big 12 schools move on to the larger conferences, but Iowa State and Baylor are left behind. There will be pressure from Iowa to force its state partner into the new Big 16 (or whatever it will be called), but Kansas is strong enough to bring Kansas State and Missouri along, while Syracuse's overall athletic prowess is more potent than Iowa State's. Baylor's lack of prestige, when compared to Texas Tech and even Texas Christian, puts the Bears in a no-win situation.
The Mountain West Conference does increase its national presence, however, by picking up Baylor and welcoming back Texas Christian. That ensures the Mountain West has 12 teams and can therefore hold a conference championship game. Those schools are:
San Diego State
You'll notice that Brigham Young is not there, and that is not a typo. That institution announced almost a year ago that it is leaving the MWC, becoming an independent in football and participating in all other sports as a member of the West Coast Conference.
One school could blow up this entire super-conference outline: Texas. The Longhorns could pull a BYU and opt to go independent in football while playing all other sports against other schools in a second-tier conference. I'm not sure how well that will be received by Texas' nationally ranked and respected basketball and baseball programs, so the guess here is that Texas moves west.
You should remember as you digest these projections that all of it is based on one national athletic powerhouse moving to another conference and a second national powerhouse doing the same. If, however, Texas A&M and Texas remain where they are (and I'm not betting on that), then no super conferences are formed.