I don't think UNLV is a realistic near-term option for PAC-12 expansion simply because they have not been good in either college football (basically ever) and college basketball (in almost 30 years). UNLV has been investing in those programs, but I would think some significant improvement would have to come first.
PAC-12 expansion would have to be about solidifying the conference as far as its football product. I'm not sure UNLV helps much, even though a ton of football recruits come from Las Vegas. The thing to note is that Las Vegas is already right in the middle of the PAC-12 footprint.
If the PAC-12 does end up expanding, I would think it would have to involve something like Texas or another Big 12 school.