In college sports realignment circles, there has of course been much discussion about how ground-shaking it would have been for the Texoma 4 to leave the Big 12 for the "Pac-16", but not much about the nuts and bolts of what might have occurred after. Here's my attempt. I don't claim it to be authoritative, so feel free to come up with your own timeline of events.
The TL;DR version:
FBS (2020)
The full version:
In June of 2010, after Colorado declares it will split for the Pac and Nebraska for the Big Ten, the Texoma 4 (OU, OSU, TT, UT) do indeed decide to join the Pac to make a Pac-16. Texas A&M was supposed to be along for the ride, but they would have none of that, leading to the Pac's selection of Utah to replace them. This occurs in spite of Baylor making an arse of themselves in attempting to either block the move or supplant another school to come with. The negotiations between the Texoma 4 and the Pac drag on into July, so while Colorado and Nebraska will depart the Big 12 in July 2011, the Texoma 4's move is delayed until July 2012. Incidentally, this also gives A&M time to make arrangements with the SEC, and Missouri is chosen as the SEC's 14th school as in our timeline.
With the Big 12 down to 10 schools by the 2011-12 season, and with 6 schools to depart the following season, the remaining 4 (Baylor, ISU, KU, KSU) scramble to retain their relevance. This precipitates an earlier separation between the Big East football-first and basketball-first schools. The ACC moves up their invite of Pitt and Syracuse to join in July 2012, and the remaining 6 Big East FB schools plus incoming TCU agree to join the Big 12 in July 2012 along with UCF from CUSA. This leaves the Big East as a non-FB circuit. Notre Dame equivocates for a while longer before deciding to join the ACC for non-FB sports in July 2013. The Big East adds Xavier and Creighton for 2012, and Butler is selected to replace ND in 2013.
Big 12 (2012-13)
East: Central Florida, Cincinnati, Connecticut, Rutgers, South Florida, West Virginia
West: Baylor, Iowa State, Kansas, Kansas State, Louisville, TCU
The moves made by the power conferences cascade down into the lesser FBS leagues. The Mountain West Conference is hit particularly hard, losing their top 3 FB schools (Utah, BYU, TCU) in the span of 2 seasons as in reality. Boise State is recruited from the WAC for 2011, as are Fresno State, Nevada, and FB-only Hawaii for 2012. This is a heavy blow for an already weakened WAC, though it is actually able to linger as an FBS conference with 7 FB schools for a year longer than our timeline. Meanwhile, CUSA shakes off the loss of UCF for 2012 by stealing FIU from the Sun Belt.
The Pac-16 is an instant success in 2012, arranging itself into 4-team "WAC-16" style pods that rotate between divisions and actually work geographically. Feeling inadequate, the Big Ten and SEC desire further expansion and aggressively probe the ACC for weakness. Rumors of Clemson and FSU negotiating to defect to the SEC and the tremendous turnover in the Big 12 and Big East combine to convince Tobacco Road that the ACC is next in line to fall. As such, schools begin seriously exploring options. Though not without regret, UNC agrees to join the Big Ten for 2014 along with conference mates UVA and UMD as well as Rutgers from the Big 12. Abandoned by their top rivals, NCSU and VT quickly agree to accept invites from the SEC to join in 2014.
Though the ACC is reeling from these announcements, the rumors surrounding Clemson and FSU turn out to be just rumors. Like the Big Ten, the SEC is actually looking for new markets. So the ACC retains a solid FB core, but as they will be down to 9 members plus non-FB ND, expansion is in order to replenish their numbers. The unhappy hodgepodge that is the Big 12 is at severe risk of being left out of the CFP/NY6 negotiations. Thus the ACC is free to take their pick of the Big 12. Seeking parity with the other 3 leagues that will be at 16 schools by 2014, 5 schools are initially invited: Cincinnati, Kansas, Louisville, TCU, and West Virginia. These adds are made possible because the ACC is already losing most of the hardcore opposition to schools that are perceived as of lesser academic quality.
Notre Dame sees that the wind is quickly blowing toward a P4 champs-only CFP and wisely commits fully to the ACC for 2014. The 16th and final member added for 2014 is dark horse Houston from CUSA, narrowly selected over UConn due to better football, because the league wants a travel partner for TCU, and also due to lingering bad blood over the lawsuit spearheaded by UConn against the ACC in the previous round of realignment. Like the Big Ten and SEC, the ACC emulates the Pac-16's rotating pod setup to operate within existing NCAA rules while still permitting all FB teams to play one another at least twice within 4-6 years.
The Big 12 has only 6 members remaining for 2014 (Baylor, ISU, KSU, UCF, UConn, USF). Despite the heavy losses, the conference is optimistic about an autobid in the new NY6. The Big 12 draws more deeply from the usual pool of backfill for power conferences, CUSA. ECU, Memphis, SMU, and Tulane are excited about joining an AQ conference. Temple is also invited as a full member for 2014 and Navy as a FB-only member for 2015. Unfortunately, as the Irish divined, the CFP ends up as P4 champs only, and no NY6 bowl autobid for the Big 12 is forthcoming. There is, however, a general G5 autobid, and as a self-proclaimed "P5" conference, the Big 12 feels confident about qualifying every year.
Big 12 (2015-16)
East: Central Florida, Connecticut, East Carolina, Memphis, South Florida, Temple
West: Baylor, Iowa State, Kansas State, Navy* (FB only), SMU, Tulane
The trickle-down effect of realignment has naturally hit CUSA hard. After losing the better half of the conference, they look to the Sun Belt (UNT), WAC (LT, UTSA), and recent FB startups in large markets (ODU, UNCC) to restock for 2014. The Sun Belt has quietly been expanding over the past few years, mostly by promoting FCS schools. They manage to reach 12 members by 2014 and hold their first CCG. With the demise of WAC FB after the 2013-14 season, Idaho and NMSU are left without FB homes. As there is no room in the Sun Belt here for FB affiliates, Idaho admits defeat and drops to the FCS Big Sky for 2015, while NMSU clings to FBS independence.
CUSA (2014-15)
East: Charlotte, FIU, Marshall, Old Dominion, Southern Miss, UAB
West: Louisiana Tech, North Texas, Rice, Tulsa, UTEP, UTSA
Sun Belt (2014-15)
East: Appalachian State, FAU, Georgia Southern, Georgia State, Middle Tennessee, Western Kentucky
West: Arkansas State, Louisiana-Lafayette, Louisiana-Monroe, South Alabama, Texas State, Troy
NFB: Little Rock, Texas-Arlington
The MAC and MWC evolve in a manner similar to that in our timeline, with the exceptions being that Temple FB sticks around in the MAC for 2 more years and SJSU/USU don't join the MWC until 2014. As in our timeline, things mostly quiet down after 2014. UConn eventually gives up on the Big 12 and joins the Big East in 2020. The Big 12 replaces them with Tulsa from CUSA, which snags FAU from the Sun Belt, which taps Coastal Carolina from the FCS Big South. Thus we arrive at the present alignment (see image at the top of the post).
For the ACC and SEC, which only have 8 conference games in FB, the pods rotate between divisions in a 3-year cycle. The teams with the same color (green vs. blue) in a column are protected crossovers, while the teams with the same font style (normal vs. italics) in a column are alternate crossovers, which are played when the team shares a division with its protected crossover. The Big Ten and Pac-16 have 9 conference games, and their pods rotate in a 2-year cycle, with East and West never sharing a division (same for North and South) and no protected crossovers.