I thought this was a really good map someone made when talk of divisions were discussed before. Darkest line are big rivals, middle ones are secondary, thinnest ones are minor ones/trophy games that might not qualify as ones.
I think you try to preserve as many of those lines (at least the darker 2) as possible. I'd probably do the following:
Maryland: Rutgers, Penn State,
Michigan State
Rutgers: Maryland, Penn State,
Nebraska
Penn State: Ohio State, Rutgers, Maryland
Ohio State: Michigan, Penn State, Illinois
Michigan: Ohio State, Minnesota, Michigan State
Michigan State: Michigan, Indiana,
Maryland
Indiana: Purdue, Michigan State, Northwestern
Purdue: Illinois, Indiana,
Northwestern
Northwestern: Indiana, Illinois,
Purdue
Illinois: Purdue, Northwestern, Ohio State
Wisconsin: Iowa, Nebraska, Minnesota
Minnesota: Iowa, Wisconsin, Michigan
Iowa: Nebraska, Minnesota, Wisconsin
Nebraska: Iowa, Wisconsin,
Rutgers
The ones underlined were the extra ones I filled out last and you could probably rearrange those however you felt fit.
One other random thought. With 9 conference games, it's possible they would not just have the other 10 teams rotate through the open 6 games. I'd probably vote for that, but if they wanted to lock schedules a bit more, there is 2 things they could do:
a) They could have a pure rotation where you play your 3 locked games every year, everyone else (5 games a year) half the time, and the last conference game could be scheduled after the last season with the idea of balancing schedules to the best that they can. Say Northwestern looks really strong, but is scheduled to miss the top 3 other teams. They would be scheduled with one of them while someone with a heavy schedule could be given a predicted easier opponent (note: home and away does complicate this approach)
b) it is possible to have a set-up where you play 3 teams every year, 4 teams 3 of 4 years, and the other 6 team half the time.