I think all the conferences should have a guaranteed spot in the playoffs. Twelve seem like it would be close to ideal, 10 conference champions and two wild cards, top 4 seeded teams get a bye and first two rounds at higher seed's stadium.
I know many don't like the idea of giving all G5s a berth, but I think you need to for a few reasons. First, some of the small conference teams could surprise -- think a Butler equivalent (the first time they made the Final Four) -- and they deserve the opportunity.
Second, and to a degree this goes along with the first reason, it isn't fair if you have a couple of G5 teams that look fairly equal and leave one out. Typically this would likely be someone like an MWC and an AAC team, maybe Boise and Houston, but this is exactly when a "Butler" type of team would be left out.
Third, you do it to give every team a chance at the championship and this should help provide more parity in college football. I agree, it won't ever be an even playing field, the blue blood schools will always have advantages and being in a major conference will give P5 teams more money. OTOH, knowing that small conference schools will be in the playoff will drive more TV viewers to at least some of their games, particularly conference championships, allowing them to earn more on their TV contracts. It could drive even more attention to some of the conferences, again the MWC and AAC in particularly, if you can get a Boise State type of team to win the championship occasionally.