There aren't enough non-conference games to accurately gauge conference strength, so all the conference champions should be in. This means an 8 team playoff makes the most sense. That provides more wildcards than I would prefer, but I'd rather a conference champion not be excluded.
If the SEC starts the year with 12 teams in the top 25, they will probably end the year with 12 teams in the top 25 because people will say all their losses were to ranked teams. If the few unranked teams beat a ranked SEC team, they will become ranked and the cycle will continue.
My ideal requires conference realignment. There would be 6 power conferences based regionally like the good old days, 12 teams in each conference. Conference champs are in. Top ranked group of 5 team is in. 1 wildcard.