One thing to think about here is what do you want the schedule to look like? Lets assume ND does agree to join and the B1G finishes out with Stanford, Oregon, Washington to get to 20. You pretty much have to go to rotating divisions (call it a mini-NFL format) so that all schools play each other semi-regularly.
You could rotate the divisions every season and play everyone not in your division twice ever six years, which would work great with 9 conference games, leaving you 3 non-cons to schedule. Something like:
Pacific
USC
UCLA
Oregon
Stanford
Washington
Great Plains
Wisconsin
Iowa
Nebraska
Minnesota
Illinois
Great Lakes
Ohio State
Michigan
Michigan State
Purdue
Indiana
Atlantic
Penn State
Notre Dame
Northwestern
Rutgers
Maryland
Pretty good competitive balance across the board with something like that.
You really can't protect rivalries across divisions at 20 schools, I assume ND would want to play USC every season, but I don't know how you make that work - you can't only two two divisions or it's basically two separate conferences. Notre Dame being in a division with NW/Rutgers would be good trips every year to NYC/Chicago.
You could also add Colorado instead of Oregon, as an example, then put Colorado in the Great Plains Division, move ND to the Pacific, and Illinois to the Atlantic, but I'm certain the TV partners wouldn't like that cuz the Atlantic division looks pretty light in that scenario.