Boise State is NOT a top tier team nor should they be a potential target. Their best chance to move up came during the Chris Petersen run in the mid to late 2000's. There are only 1.7 Million people in Idaho and it's disparately located. Once your bar is Boise State, then you've conceded that you're a G5 league.
For sure invites should be BYU and the 'Metro' AAC Schools:
BYU
Houston
Memphis
Cincinnati
BYU is a national brand. The other 3 are in metro areas close to existing Big 12 teams. Having Kansas, Memphis, Baylor, Cincinnati, and Houston together would make the Big 12 the premier men's basketball league in the country.
Second Tier should be SMU and the directional Florida AAC Schools:
UCF
USF
SMU
I would have put UCF and/or USF in the top tier, but their location is so far away from the other Big 12 schools, that the travel burden for non-revenue sports is going to be a beast. I think both would be good additions financially, but I don't know if either moves the needle enough to justify the additional cost.
SMU would be the next alternate if a deal with Houston can't be made. The main reason I would choose Houston over SMU is that you already have TCU (and to an extent, Baylor) in the DFW metroplex, but with A&M gone and Texas leaving, there is no Big 12 presence in Texas south of Waco, which seems kind of crazy when I type it out.
Third tier should be the Mountain West and maybe Rice:
UNLV
San Diego State
Colorado State
Rice
Mountain and Pacific Time Zones are a ratings killer and I think if you have to dip past the AAC to either Mountain West or Conference USA, it would be a death knell for the Big 12's credibility as a "Power" conference, but UNLV could be a program on the rise. They play in the Raiders' new Allegiant Stadium and Las Vegas has almost overnight become a sports destination, adding an NHL, WNBA, and NFL team in the past 4 years. They also host the NBA's Summer League with indications they're next in line for an NBA franchise. Having a member school in that location could be an alluring thought in the near future. The others are just backfill located in/near metro areas (San Diego, Denver, Houston).