I don't think the Big 12 would go with a North/South split if we added, say, Cincinnati and Memphis. The state of Texas is the lifeblood of the conference, and putting all of those schools in the same division would result in a pretty severe talent imbalance (like we saw before). The Pac-12 worked the geography to split the state of California in their divisional split; I think the Big 12 could sort of do the same.
East
Baylor (TCU)
Cincinnati (Kansas State)
Memphis (Kansas)
Oklahoma (Oklahoma State)
Texas (Texas Tech)
West Virginia (Iowa State)
West
Iowa State (West Virginia)
Kansas (Memphis)
Kansas State (Cincinnati)
Oklahoma State (Oklahoma)
TCU (Baylor)
Texas Tech (Texas)
Parentheticals are the permanent crossover rivalries. Play the 5 other teams in your division, the permanent crossover in the other division, and 3 of the remaining 5 teams in the other division each year.
I think it would look something like this.