I think it is now a when UT and OU leave for the SEC there aren't many if any good options for TCU. I am not sure this is a bad thing with the NIL and other NCAA issues for the purists. Basketball does help Baylor, but it can get overlooked.
The Big Ten will select who they want to join if they expand. Notre Lame is number one on that list! Mizzou and I would guess UVA or UConn are also high on the list. Boston College might also get a consideration. KU is very unlikely, but not out of the question. They are the premier conference!
The ACC likely looks to backfill. I would imagine WVU has to be high on their list. After WVU it depends on how many openings they want to fill. If it is 2 they very well could look at Texas schools. Those 2 schools might be Baylor and TCU. My guess is that TCU is probably 3rd or 4th on their list.
The Pac 12 will not take Baylor! They want nothing to do with religious schools that want special treatment! They made it clear years ago when UT almost joined. All Texas schools struggle when it comes to California travel restrictions, but I think TCU is in better shape then some would imagine when it comes to the Pac. Playing Pac teams shows a commitment. If the Pac goes for 4 I think the choice is KU, KSU, OSU, and TT. Who knows if that holds. I think TCU is number 5 on their list and maybe 6.
The other conference of note in this entire situation is the Big East! They have Creighton in Nebraska so adding KU and KSU might make some sense for them. I also think the Kansas schools could be intrigued with the basketball. Iowa St. might even consider it. I could also see TCU enjoying a spot in the conference though football might not look like it does today.
The best hope for TCU is that if UT and OU leave that the Big Ten selects some of the big fish. A selection of 2 of the following (Mizzou, Notre Lame, BC or UVA) helps TCU. That likely opens 1 spot in the SEC and possibly 3 in the ACC.
The other option for TCU might be to align with SMU, UH, Rice, Tech (if available) and Baylor to find other schools around Texas and restart the the Southwest Conference. University of North Texas, UTSA, and UTEP might be options. The schools that are no longer a part of the conference will whine, but it could be the best option.