When Texas and Oklahoma leave, who seriously brings in viewership out of the Big12? What brand or brands can carry that conference?
At least with USC/UCLA leaving the PAC, the PAC still has plenty of markets and brands to carry it on.
To answer your question:
BYU, Oklahoma State, Texas Tech, Kansas Basketball, UCF, Cincinnati, West Virginia, and Iowa State all have decent followings. I don't have much confidence in the other Big12 teams carrying much of a following though. Baylor has had a lot of success lately but long-term they have not shown they are a solid program. Throughout the first 15 years or so of the Big12, they were the Vanderbilt of the Big12. Houston brings nothing. TCU had that nice run under Patterson but has been meh since he left. Texas Tech isn't that great of a football team but they are not bad either. They always seem to be just average but they do have a great atmosphere and fan following.
Kansas State has done well in the past but not sure it is enough to drive a brand.
BYU, Oklahoma State, and West Virginia are the three biggest brands IMO. Those three will probably drive the league. You also have Baylor, UCF, and Cincinnati all being up right now to help as well which helps but I am not sure they can stay at their current level long-term.
Kansas State, Iowa State, and Texas Tech all have dedicated but small fanbases. They fill their games, have great atmosphere, etc. However, they are NOT big brands as you said. They are decent brands but not big.
With what they have, they will be an alright league but definitely behind the Big 3 right now (ACC, B1G, SEC). I think we are heading towards a Power 3, a mid-tier 2, and then a G5. (Big12 and Pac12, if it survives are not G5 but they are not in Power2 either).