The consensus indicates the two newest members to the Big XII will be BYU and UCF, which are better than a number of schools in the Power 5 conferences when it comes to football.
The conference would be wise to go ahead and get two more to keep the conference well ahead of the AAC (which would be crippled) and on par with the Pac12 (might argue it will be superior). Who that might be is a big guess, but I would place my bet on one from the east and from the west for geographic scheduling purposes if nothing else.
My two cents on expansion in getting to 12 schools would be adding the following:
Cincinnati and Houston (Both have way more pros than cons compared to other candidates)
When all said and done, the new Big XII, would probably be stronger in football than the Pac12 and even the ACC (Always Clemson Conference). Furthermore, the conference would get a decent boost in basketball and baseball.
Side note when it comes to BYU with baseball....Admission means you have to play on Sunday's.