I'd think the pecking order would be:
1) ND
2) if ND wants assurance that Stanford joins, to be a catalyst for ND joining, that's the next best school.
At this point you're at 18, with 3 west coast schools and ND, who obviously would like travelling to the west coast, so they'd be okay making 2 trips per year out there. Hell they already do it this year.
If you go to 20, I would still take 2 more west coast teams. That's just more west coast inventory to sell. Grabbing a UNC/UVA doesn't get you more games in the west coast.
Imagine something like this:
Michigan at UCLA -- 8:00 Fox
Nebraska at Stanford -- 10:00 FS1
Wisconsin at Cal -- 10:30 FS2/BTN
BTN getting content at 10:00/10:30 is something they've never had before. Being able to have games on BTN at the Noon, 3:30, 7:30, 10:30 slots is great programming. Same for FS1/FS2.