I've pitched this before, but if the Pac chose to add 4 teams (let's say ISU, KU, OSU and TT), you can schedule it quite nicely thru a pod system that ensures a schedule that would allow all previous Pac-12 members to only trade in two previous in conference foes for two new ones, allow one game in each region (Northwest, Cali, SW/Rockies, Midwest) ensuring every team gets to play in California every year, while all regional rivalries are protected.
NW Pod (Washington, Washington State, Oregon, Oregon State)
Cali Pod (Cal, Stanford, USC, UCLA)
SW/Rockies Pod (Arizona, Arizona St, Utah, Colorado)
Midwest Pod (Iowa St, Kansas, Oklahoma State, Texas Tech)
Play the three in your pod yearly, and two in each other pod, alternating home and away and alternating opponents every two years. Allows you to play every school both home and away within the span of four years.
The other option would be to go straight east/west, play the other seven schools in your division, and alternate thru the other 8, but that would be far less optimal.