I wish I could credit where I found this but not sure... I attached the set up to this post which should giver you a good idea of what I'm doing: 5 "premier conferences" and 5 "lower conferences" with balanced divisions. The worst teams are relegated and conference champs are promoted.
I'm 5 seasons in on this and the issue with this though is that A: it doesn't add parity too much; mostly teams are promoted and the relegated the very next season or the teams you would expect (Wake Forest eg) are relegated and stay down. The bump a program gets in recruiting for being promoted isn't too awfully profound AND if there is a bump obv those players don't have an impact for years.
ALSO, fortunately, this has only happened once. Since I don't mod a "lower conference" team can still go undefeated and win the natty which is very problematic.
Overall I like the system though. I'm considering promoting/ relegating 2 teams per season now or making a promotion/relegation a 2 season thing, like a team isn't eligible for promotion until they've played 2 seasons in lower league.