The Big 12 should not even remotely consider academic prestige when expanding. There's zero reason to.
I would love to see Tulane have success in football and basketball and grow its fanbase. It would then become a possible expansion candidate (and we could do a lot worse than New Orleans as a conference).
Unless, of course, you're one of the ten university presidents who 1) are the only ones who have a vote, 2) are first and foremost academics serving in an academic capacity, and 3) actually care about the quality of universities with which your university is associated. As an ISU alum, and as a professor at another Big 12 school, I certainly care about the academic stature of the universities in the Big 12... especially after losing four different AAU schools and replacing them with WVU and TCU. I don't see how that line of thought can change so drastically once one becomes a university president, since most of presidents were teaching and researching as university profs in the not-so-distant past.