As long as BYU is an independent team there will be talks about them rejoining the Mountain West, or on the BYU side getting that elusive invite to the Big 12. Commissioner Craig Thompson had his state of the conference address on the first day of Mountain West media days and he went over plenty of different topics, including conference realignment thus sparing a few questions about the former conference member.
The new college football playoff will have a key role in what BYU's future will be in regards to any conference, and Thompson is in no rush to be proactive in realignment. In fact, the commissioner wants things to "marinate" and settle down before making a move; that being said, Thompson will not be blind if schools have any interest in joining the Mountain West.
This brings us to BYU, who has multiple Mountain West teams on its schedule including future games against Hawaii, Nevada, UNLV and Boise State. Despite playing almost half of the Mountain West, Thompson has no "nailed down discussions" about BYU coming back to the conference. The college football playoff may factor into BYU's possible future conference affiliation: if the Cougars can find a way to land an at-large spot on its own, they are more comfortable being independent, whereas if they are unable to find a way in, they may be looking to join a conference with a guaranteed way in.
As of now, any independent not named Notre Dame has no way to automatically qualify for a New Year's Day Bowl game. The non-power conferences have an automatic spot for whomever is the best team according to the selection committee, so access is greater for these teams. BYU see's themselves as a step above these other schools. BYU will find out in the next year or two where they stand since they are expected to have very good teams during that stretch.
One way BYU could make a move back to the Mountain West is if they realize that, despite having an undefeated or one-loss season, being independent gives them no chance to make the playoff, or a New Year's Day Bowl, while a team from these non-power leagues have a worse record yet still make it to a profitable post season game. The latter could be the real tipping point towards BYU feeling that they need to join a conference; it would be extremely frustrating if BYU has excellent seasons yet Utah State, for example, gets into the Cotton Bowl with a worse record, and possibly even a loss to BYU.
It could still be years before BYU even thinks of rejoining the Mountain West- or moving away from independence in any way- as they navigate the new playoff landscape. That makes Thompson's assumption to let things "marinate" on any conference realignment within the Mountain West seem wise.
It would take a near miracle, or at the least a giant shift in the college football landscape, for BYU to ever return to the Mountain West, but it wouldn't be Mountain West media days without the Cougars getting a mention.