I think it's a bad assumption to think that Texas would necessarily follow OU if OU left. It's part of that media superconference bullshit.
That's what makes this so interesting. Texas as the wild card in this whole thing is what creates so much uncertainty. All this talk of Texas and Tech to the PAC 12 right now is probably the OU equivalent of A&M's "gone to the SEC immediately, done deal" thing a couple of weeks back: 99% coming from OU in an attempt to force the dialogue.
OU and OSU *could* go PAC 12 without Texas right now. PAC 12 might go for that to get into the heart of Big 12 country, the Dallas market, and increase the odds of eventually landing Texas.
What would then be Texas' best option? They're going to have legislative pushback if Baylor or Tech gets left out, but who knows whether it will be enough to affect anything. They might not like the perception of having their hand forced by OU, especially when they still have a pretty sweet deal in the Big 12 (which would presumably get even sweeter with the new teams that would be brought in). So maybe they stick around and bring in BYU and Air Force and Houston while they survey the landscape over the next year or two.
Don't really see Texas to the SEC. That would essentially be them following aTm in, and doubt they'd be interested in giving aTm that win. Also, SEC academics seem lower than Texas would want.
Texas to the Big 10 could be likeliest ultimate scenario, but still don't see that immediately, until Texas can milk some of this revenue before going to the Big 10's revenue sharing.
If Texas and Tech do join OU and OSU in going PAC 12, then the question becomes whether this precipitates any other conference jumping to 16 teams in the race to 64. I would actually be surprised if the Big 10 or SEC immediately acts to try to get to 16, unless Big 10 thinks Notre Dame is now in play. Both those leagues are guaranteed to be one of the superconferences when they choose to go to that, so they don't actually have to worry about expanding until that actually happens.
The real issue will be whether the Big East and/or the ACC believe that they have to act to get to 16 right now to position themselves to be the the 4th superconference. Both conferences would be vulnerable to losing teams to either the Big 10 or the SEC when either of those conferences decide to expand. The Big East is a more solid football conference right now, but they'd likely have to add ACC teams to get to 16, and ACC teams aren't likely to leave for the Big East until the SEC poaches a couple of ACC teams and puts its future in doubt. The Big East would probably try to take advantage of its existing affiliation with Notre Dame, but like the Big 10 and SEC, Notre Dame probably won't act until it's decided to go to the 64 team superconferences across the board.