I have a romanticism to having the Big Sky absorb WAC leftovers, though I guarantee New Mexico State would go back to the Sun Belt.
However, and I must emphasize that I strongly believe this...
(1) The Pac-10 will not go to 12 with Mountain West schools for at least 25 years. If they go to 12 before then, just totally fuhgeddaboutdit. This assumes that they go to 12 at all, for which I have more serious doubts.
(2) The Mountain West has no earthly reason to go to 12. If they go 10, it's only because either Fresno finally meets their demands, or TCU needs a travel rival, or by some fluke of nature, Boise State actually maintains their football level for 5 years. I don't see an expansion for at least 5 years. Frankly, I think they see Boise as a last resort.
(3) Therefore, there's going to be a WAC. Oddly, the WAC might be motivated to go 12 for reasons of cutting travel costs.
I think the pressure on Montana to move up becomes greater over time. If they do, this fractures the Big Sky, loosens a couple other schools for WAC membership, and remnants of that + the Great West becomes the replacement 1-AA conference. I'm curious to see whether Sac State (possibly yes) and Portland State (possibly not) still have football programs by the time that happens... if it happens, of course.
I'm not going to rate who moves up, because I also believe that 1-A will at least maintain the 15,000 attendance number for schools trying to rise from 1-AA. Montana obviously surpasses this, and Montana State is closest to being the next school to do so. Sacramento State and Portland State obviously have the potential, and Sac might be devoting the resources, while Portland State is beginning to focus on a basketball arena (the basketball team finally won a Big Sky regular season, and they finally started to sell out 1,500-seat Stott Center) rather than solve a nagging football attendance problem that is "growing" rather than improving. Northern Arizona built themselves into the wrong dome design, otherwise they have potential as well. Eastern Washington expanded their football stadium before last season, and there were a couple gates good enough to indicate that they didn't expand enough, and they have space, but they have to fill for Sac State the way they do for Montana. Weber State is not in the right market, though they have the facilities. Idaho State has no chance.
Thing is, I think UC Davis has a better chance to make a move than Sac State. Their football facility proposal is better than Sacramento's. Their basketball facility is already better, though the proposed new Sac facility probably trumps it. They are close enough to Sac to take a chunk of that market PLUS have more respect in the broader reaches of Northern California, and they are far enough away to not be subject to commuter school issues. Now, if the state would have seen fit to throw some resources to Chico State, we wouldn't be having this conversation, but I digress...