If you keep the playoff to 8 teams, you’ll get all of the undefeateds plus the best 1- (and maybe 2-) loss teams. Good enough for me.
It’s a lit better than relying on computers and subjective polls. Let ‘em play it out on the field. Works for Div 1-AA (now FCS), Div 2, and Div 3.
Eight teams would work. Along those lines, Brent Mussburger suggested that the top 8 conference champions be picked for the playoffs. That would also work because this year the MW and WAC both deserve to have their conference champions in and the Big East does not.