With the Big 12 inviting TCU and TCU accepting membership, it looks like the Big 12 has at least stabilized some (pending Missouri's decision, which I hope and expect to remain with the Big 12). Additional candidates are likely to go to 12 if not 14. I think this would be a good time to look at the organization for division formation. Previously with 12 teams, it was a North vs South with the line drawn between Kansas and Oklahoma. This geography would be hard to re-establish, and an East-West split would also constrain current rivalries as well as future additions. Witness the SEC with 13 teams, 7 West and 6 East. Either the SEC will have to redraw its division if it adds Missouri, or be constrained to take a team to the East (West Virginia? Virginia Tech? Miami? A North Carolina team?). And then what would have to change when it feels 16 teams is needed? Redrawing division lines while preserving traditional rivalries will be difficult for any conference. Thus the Big 12 should give serious consideration now, before it has divisions, as to how the divisions should be drawn to accommodate future growth.
While conference realignment appears largely about money, the best thing about college sports is rivalries and these must be preserved in any division alignments. Thus if rivalries are to be preserved, why not build divisions around rivalries? Currently in the Big 12 there are the following rivalries, from oldest to youngest (rivalries with Texas A&M not included).
Kansas & Missouri (Border War since 1891)
Texas & Oklahoma (Red River Rivalry since 1900)
Kansas & Kansas State (Sunflower Showdown since 1902)
Oklahoma & Oklahoma State (Bedlam Series since 1904)
Texas & Kansas State (Chisholm Trail since 1913)
Iowa State & Kansas State (Farmageddon since 1917)
Texas & Texas Tech (Chancellor's Spurs since 1928)
Oklahoma & Missouri (Peace Pipe since 1929)
Baylor & Texas Tech (Texas Farm Bureau Shootout since 1929)
Iowa State & Missouri (Telephone Trophy since 1959).
As can seen in the above, keeping all the rivalries within one division will not be practical. There are two kinds of rivalries, intra-state and inter-states. Since there are more inter-state rivalries, and rivalries should be annual games, it makes more sense to build divisions based on inter state rivalries and keeping intra state state rivalries as an annual inter-division game. With this format the following intra-state rivalries will split the conference into two.
Oklahoma & Oklahoma State
Kansas & Kansas State
Texas & Texas Tech
Around this we will add inter-state rivals, keeping interstate rivals in the same division. In the South we have Oklahoma & Texas (so Oklahoma and Texas should be in the same division). In the North we have Kansas & Missouri and Missouri & Oklahoma (so these 3 teams should be in the same division). The result is Division A: Kansas, Missouri, Oklahoma, and Texas with Division B: Kansas State, Oklahoma State, and Texas Tech. Given that both Iowa State and Missouri do not have in state and in conference rivals, and they are rivals as well, the two will be matched as cross division rivals. Now Division B has Iowa State, Kansas State, Oklahoma State, and Texas Tech. There remains two teams for placement, Texas Christian and Baylor. Texas Christian has an 106 games rivalry with Baylor and a 51 games rivalry with Texas Tech. Baylor has a 67 games rivalry with Texas Tech. Thus Texas Christian and Baylor should be in opposite divisions, with Baylor in the same division as Texas Tech.
Thus we end with:
Division A: Kansas, Missouri, Oklahoma, Texas and Texas Christian.
Division B: Baylor, Kansas State, Iowa State, Oklahoma State, and Texas Tech.
The balance of power appears to favor Division A (with current upper powers Oklahoma and Texas and mid powers Missouri and Texas Christian) over Division B (with current upper power Oklahoma State and mid powers Kansas State and possibly Texas Tech). But power rankings will change from season to season. Besides, with cross division rivalries, all the strong teams from one division will play against the other division teams as well. With expansion to 12 or 14, addressing power imbalance can occur.
Each year each team should play its cross division rival to open conference play. Playing your cross division rival first because should a set of cross division rivals end up being division champions, their rematch in the division championship would come after all conference plays have occurred. It would also be a great way to start conference play.
Baylor opens with Texas Christian.
Iowa State opens with Missouri.
Kansas opens with Kansas State.
Kansas State opens with Kansas.
Missouri opens with Iowa State.
Oklahoma opens with Oklahoma State.
Oklahoma State opens with Oklahoma.
Texas opens with Texas Tech.
Texas Christian opens with Baylor.
Texas Tech opens with Texas.
Another benefit to divisions based on rival is that travel cost for each team also averages out more evenly. In the old Big 12 division the southern teams had a shorter distance to travel (Oklahoma and Texas) while the northern teams had longer travel distance (Colorado, Iowa, Kansas, Missouri, and Nebraska). In a division of rivals both divisions have Kansas, Oklahoma, and Texas with one also has Iowa while the other Missouri. I also see this as enhancing the smaller state's presence within more populous Texas for exposure, fan building, and recruitment.
With a conference built on rivalries, any additions to the Big 12 should thus be recruited as rivals set. For instance Louisville + Cincinnati (or West Virginia), BYU + Boise State (or West Virginia), Tulane + Southern Mississippi, etc. The rivals division is practical for 12 or 14 teams arrangement. It will need a bit of tweaking for 16 teams though.
Finally, each conference teams should try to cultivate annual extra-conference rivalries. For instance Iowa State and Iowa, Texas and Texas A&M, Texas Christian and Southern Methodist, and Texas Tech and New Mexico. Of the expansion candidates, it would be BYU v Utah (or Boise State), Louisville vs Kentucky, WVU vs Pittsburgh, and Tulane vs LSU for instance. These inter-conference games would highlight the conferences against each other at the beginning of the season, whereas bowl games will highlight them at the end.