IMO, going to 15 is the best number. If you added 1 team and went to 15 then you could go to three 5-team pods. You'd play each team in your own pod every year, and then alternate years vs. the other pods. This would give you the 9-game schedule, and you'd never go more than 2 years without seeing every team in the league.
For example:
"New" Pod: Penn State, Maryland, Rutgers, Nebraska, Oklahoma
"Central" Pod: Ohio St, Michigan, Michigan St, Indiana, Purdue
"West" Pod: Wiscy, Iowa, Minnesota, Illinois, Northwestern
These 3 pods are "fairly" balanced... I know the "West" is the easiest". But you can never get perfect balance.
Take Penn St for example: Every year we'd play Maryland, Rutgers, Nebraska & Oklahoma. Year #1 we'd play the "Central" pod, year #2 we'd play the "West" pod, year #3 Central pod ..... and so on.
We'd still see great teams every year (Nebraska & Oklahoma) and we'd see Ohio State, Michigan, Michigan St on alternating years.
This would also keep pretty much all of the big time rivalries played EVERY year.... Mich/MSU, Mich/OSU, Wiscy/Iowa, IU/Purdue, PSU/UMD, PSU/Rutgers, Illinois/NW, Minn/Wiscy.... all would be kept as yearly games.
The only problem would be the conference Champ game. How would you pick the teams ???