With 20 teams you could have four 5-team "pods". You would always play the other four in your pod, and would rotate through the other three pods, playing one each year. The big ten gets their 9th conference game that way. The conference championship would be easy, as the pods that play each other would act as the "division" for that year.
Obviously the issue is some schools might not like only getting a team like Ohio state at home once every six years (or in general only once every three years). But I'm sure the additional tv revenue would keep those complaints down.