The Pac 12 is in a tough spot when it comes to expansion with no clear candidates. If you analyze them by football, academics, and market you get the following.
Football:
SDSU
Boise St
Nevada
Fresno St
SMU
Academics (R1)
UNLV
Nevada
New Mexico
Colorado State
*SDSU is R2 but striving for R1
Market
UNLV (Vegas)
SDSU (some connection to SoCal after losing LA)
SMU (gets you in Texas and Dallas, but how many eyeballs are you really getting?)
UNM and Boise St are intriguing markets in the long term as those state's populations continue to grow.
SDSU seems like the clear favorite especially if they can achieve R1 status in the near future. I think adding SMU for Texas is foolish and won't provide what some are projecting. Houston would have been the add before they got a Big 12 invite. UNLV is atrocious at football. If they could steal Nevada's football program, they'd be a lock. New Mexico's football program is on life support as well. Boise St is purely a football play. Academics are lacking and their other sports programs are off the radar. I don't get the Fresno St appeal outside of football, but maybe somebody more familiar with the inland empire could educate me.