I believe it's Arizona.

It's a good take. I can see the Arizonas being tempted to jump. Not sure it starts the dominoes though.
If the Pac-10 lost ASU and UofA, i think they would add SDSU and SMU and stay at 10.

I don't think UW would come to the Big 12. For several reasons. Let's say they O and UW come out of desperation, the Big 12 would LOVE to have them.

Putting yourself in those schools' shoes, the last thing they'd want is to be outnumbered voting wise. They would demand a stronger voting bloc with more like minded schools. That's where I'm confident UW and O are essentially a package deal with CU and Utah.

Part why you have seen strong solidarity so far.

The Big 12 would also LOVE both of those programs. For multiple reasons. All 4 is a grand slam.

At that point, you take the Arizonas as well.