Right now no matter how you slice it, at least one P5 conference is on the outside looking in with the current 4 team playoff setup. Or as we saw this past season with Notre Dame getting a spot, two P5 conferences sat out. Right now, the SEC and ACC are guaranteed a spot every year, and there has been talk before of both Alabama and Georgia both getting in. The Pac 12, Big 12, and Big Ten, OTOH are more hit or miss.
I think an expansion to 8 might be doable in the next decade. At least one team from each P5 conference so no conference is left out in the cold. Nothing lasts forever and at some point in the future instead of the Pac whatever number, it very well could be the ACC or SEC left out of a 4 team playoff picture. Such an expansion would guarantee that would not be the case. In the event Notre Dame goes 12-0 again, assuming they don't formally join a conference, this would avoid the case of 2 P5 conferences being left out. Give the highest ranked G5 team a slot. And then one or two at large teams.
1 SEC
1 Pac 12
1 Big 12
1 Big Ten
1 ACC
1 G5 team guaranteed.
2 at large teams, or one at large team if Notre Dame goes undefeated, assuming of course that they do not join a conference.