Relegation will never happen in any sport in America. Why would any current owner of a top division team ever agree to it?
In theory the big 12 might because with 24 teams they would cover more of the country, but I agree Still, maybe that only means a different answer is needed.
Suppose all the G5 banded together, and each conference expanded to 14 teams. Then suppose that each year each G5 conference sent two teams up to a super Conference, which would then leave you with a P5 like ten team super conference and a number of G5 conferences. At the end of each year one of the two promoted teams would be relegated back to their conference while a different team would be promoted.
I love the idea of relegation. But, I think it would be impossible to get anyone to agree to it. Mainly because of football and basketball.
Here's an alternative it will be impossible to get everyone to agree to: Dump conferences for all sports except football basketball and a few more "core" sports. "Regionalize" everything else. Stop chasing "national titles" in sports where, honestly, it really doesn't matter. Especially individual sports like golf, swimming, track, wrestling, etc... Play a regional schedule against all sizes of schools, and have a conference/regional tourney and be done with it. And, if some small school wants to try to dominate golf, who cares? Let them offer the scholarships and play with the big boys.