From what I heard (from a guy involved at the U in its joining the Pac-12), in years BYU was dominating Utah, BYU refused to join other conferences if they wouldn't take Utah, too. With enough people at the U having some sympathy to BYU, perhaps they'd have an interest in pushing the Pac-12 to invite us. From the U's standpoint they'd probably believe they'd be giving up a recruiting advantage vs. BYU in Utah, and would be splitting the local TV market. But I'm not positive the advantage of either to the U is all that big, and they'd be adding back the rivalry. And if the Pac-12 believes the Big 12 must expand, and that BYU is a prime candidate, the Pac-12 may be tempted to pre-emptively bring BYU into the fold. And if P5 was going to happen for BYU anyway, the perceived recruiting advantage and TV market ownership would also be going away anyway.

Just thinking out loud. I'd hasten to add the same guy who told me about BYU refusing to bolt also blatantly told me that BYU to the Pac-12 was a no-go for reasons of our religious affiliation / Prop 8. But after the Supreme Court's recent same sex marriage decision, there's less of an axe to grind, and the additional money to the Pac-12 from expansion could change its mind.

A guy can dream, can't he?