Not saying that the PAC12 is falling over themselves to add BYU, but I think we make a mistake completely ruling them out.

The Big12 has some cultural issues with BYU as well and plenty of other comparable (or potentially even stronger) options. People always talk about the Next Best Alternative. The Big12's NBA is Cincy or UCF or Memphis. If there's anything that makes them at all uncomfortable with BYU (whether that's religion, scheduling, geography, research rep, population, on-field performance, BYU wanting full payout sooner, whatever the potential issue) they can easily pivot to great options. Even Houston as a last-shot option (though that's not preferred for recruiting options)

The PAC12 has no realistic expansion options that don't include BYU. Every option is deeply flawed so they're going to have to deal with something they don't like if they expand.

San Diego St
Pros:  Great location w/ limited travel and high population, decentish quality team, 3 national championships*, Large Public State School
Cons: UCLA/USC won't like the extra recruiting competition, mediocre history beyond Coryell and Rocky Long (who just left), Tier 2 Research University

Boise St
Pros: Good at football, Public State School
Cons: Bad at most everything else, Small Population, limited history, poor academic rep (both undergrad and research), Tier 2 Research Institution

UNLV
Pros:  Good location that expands market, Tier 1 Research University, large investments into facilities, strong basketball history, Large Public State School
Cons: Downright bad at football (both historically and recently)

Hawaii
Pros: Fun location, potential for strong Polynesian/Islander recruiting, Tier 1 Research Institution, Large Public State School
Cons: Terrible travel, hurts other Pac12 recruiting, terrible facilities, bad at sports, limited fan base 

BYU
Pros: Strong on-field product, built-in fanbase across country (particularly in the West), strong history, good secondary sports
Cons: Small population base (particularly split with Utah), culture/scheduling issues, Tier 2 Research Institution

Utah State/New Mexico
Pros: ..., they're both a public state school so I guess that is something
Cons: No fanbase, small population, limited (at best success), limited investments, Tier 2 Research Institution

Air Force Academy 
Pros: Built-in military fanbase, Provides Colorado a partner if UtahSt/BYU/UNLV come in as Utah's 'rival'
Cons: Not great at football or other sports, old facilities, option offense unlikely to translate to higher level, federal justification for increased spending unlikely, no grad school research activity

So...which of those schools make the list if you are looking to add 2 (or even 4).  From my mind, there's only 3 that even have the ability to argue - SDSU, BYU, UNLV (more on potential and market than performance, but there's definitely potential there).  Certainly not as strong as Big12 options.

Pac12 options vs Big 12 options
BYU = BYU
SDSU < Houston (good programs with recruiting concerns, Houston has been better on the field)
UNLV < UCF (programs with large population bases, facilities for potential.  UNLV has more history, UCF is better.  UNLV might be more comparable to USF actually)