BYU is not getting into the PAC, the Big10, the SEC or ACC. That isn't even a pipe dream. That being said, here is my best case:
The Big XII adds BYU, CSU, UCF, and Cincinnati to get to 12 teams. I think that conference is as good as the AAC but has autonomous status for the near future, which makes it better than the AAC. The Big XII gets a footprint in Florida, Ohio/Pennsylvania, Denver, and Utah. Utah is getting bigger and better but Florida and Ohio/Pennsylvania are recruiting hot spots. That's it. That's the best BYU can hope for.
The Big XII may not be able to add UCF and Cincinnati as they may not want to leave the AAC, but it would still be better than what we are now.