A few reasons I think the Big 12 may consider a second wave of expansion:
The Big 12 may want to get bigger to fend off any future existential threats
The Big 12 explicitly stated that they may still be looking to expand
There's a lot of noise from insiders regarding a second wave

A few reasons I think Boise might be in that second wave:
Again, lots of smoke on Boise/Memphis
Boise/Memphis are behaving as if they have some inside info into the possibility
Boise is probably the strongest G5 brand/program left there, both from a recognition standpoint and on-field success
Big 12 hasn't liked WVU being on an island, but just created two more with BYU and UCF.  Boise (and USF) could help alleviate that

(Note that there's also a lot of reasons to NOT consider Boise, but I'm not going to go into all of those here)

Things that have zero bearing on Boise's fit in a second wave expansion:
Boise beating BYU yesterday afternoon