There are barely enough quality teams for a four team playoff let alone a 16 team playoff. I think 6 is the right number in terms of quality. I can see going to 7 like the NFL just did to reward the one seed. That would give you a nice first round of noon, 4 pm, and 8 pm starts.
Other sports have expanded playoffs purely for money, it has nothing to do with crowning the best team as champions. That being said in professional sports there isn’t a huge difference from the 3rd best and 9th best team in each conference. In college there is a huge difference between #3 and #6. The more teams you have in college the more you dilute the final four. Just look at BBall. You rarely see the marquee matchups.
So I would go with a 7 team playoff where any conference champion in the Top 8 gets and auto bid. Any other bids go to at larges. That allows access for the G5. It also might force ND to join as in theory they could be Top 5 and get left out. It makes winning your conference a big deal, giving meaning to the regular season.
Also the should absolutely go back to a BCS style formula. Let people seed the teams but qualifications should involve unbiased computer rankings.
So this season would likely be:
1. Bama vs (4. Notre Dame vs 5. Iowa State)
2. Ohio State vs 7. Cincinnati
3. Clemson vs 6. Texas A&M