I could actually see
UA
ASU
UO
UW
Cal
All joining us in the now Big18. Scheduling is weird, but not impossible with 18 members.
You could do 6 groups of three: For 8 conference games, you play the two others in your group every year, plus one from each of the other 6. For 9, you play two others in your group, plus another fixed one from outside your group, and then one from each other group.
You could also do 3 groups of 6: For 9 conference games, you play the 5 others in your group every year, plus two each from the other two groups. For 8 conference games you do 5 in your group plus either a rotating 1 and 2 from the other groups, or a "5 in your group, plus one fixed opponent outside your group, plus an additional one from each other group."
Stanford goes independent, and Utah gets screwed.
Scheduling works better with 14 or 16 than 18, and I'll leave "accretive" and other considerations aside, but
1. I think Cal or UA are the most likely to get the conference to 14. My money would honestly be on Cal.
2. If it's 16, I think it's Cal, and two of: UA, UO, UW and ASU. I honestly don't know which two of those are out - definitely one of the Arizona schools - after that I dunno, unless UO or UW steps on their own dick and/or the WSU/OSU politicians get involved.
I think Utah is going to get cock blocked by BYU - their only hope is for the Pac to survive in some quasi-recognizable format.
The Pac itself will survive. It may not be recognizable, but I think it will technically survive as a conference. I will remind you that the Big East still exists (although counterpoint: the SWC does not).
More schools are leaving the Pac, and many will be joining. CSU may finally get their dreamed of invite. Of course, when they get to the party, they're going to find out that the cool kids have already moved on to a bigger, better party. They'll be fine, they're used to that sequence of events by now.