If you start expanding it to 12 or 16, you'll get teams resting their starters in the last games of the year. That brings in a whole lot of other problems. The idea of 2 or 4 teams getting a bye is grossly unfair, too. Do you want Herbie, Desmond, Pollack, and the other worthless talking heads to essentially pre-determine which 2 or 4 teams get a bye to begin the playoff? If the "top 4" teams get a bye, that just continues to increase the odds that one of them will win it all. It's hard to lose a game if you don't have to play a game. You just continue to perpetuate the problem.
8 is the right number - automatic bids to all P5 conference champions, 1 G5, and 2 at large. That system will continue to put the pressure on everyone to make every game count.