Keeping in mind that the system is designed by the P5, so to get buy-in, it has to keep them with some advantage (either a bye or guaranteed access)

The 8 team is the most likely and the simplest. Every P5 champ, 1 G5 team, and two at-large.

This season that would be
1. Alabama vs 20. USC
2. Notre Dame vs 9. Iowa State
3. Clemson vs 7. Cincy
4. Ohio St vs 5. Texas A&M

12 team could work with 10 conference champs, 2 at large, and basically give the top 4 teams the week off while (most years) the P5 teams beat up on overmatched G5 teams. This could actually be best from the P5 perspective since it backs up the P5/G5 chasm in a safe way.

This season that would be
1. Alabama vs winner of (18. Coastal Carolina vs 20. USC)
2. Notre Dame vs winner of (9. Iowa St vs 21. Marshall)
3. Clemson vs winner of (7. Cincy vs NR. Buffalo)
4. Ohio State vs winner of: (5. Texas A&M vs NR. San Jose State)

or if, BYU was still in the MWC
1. Alabama vs winner of (14. BYU vs 18. Coastal Carolina)
2. Notre Dame vs winner of (9. Iowa St vs 20. USC)
3. Clemson vs winner of (7. Cincy vs 21. Marshall)
4. Ohio State vs winner of: (5. Texas A&M vs NR. Buffalo)
*though I bet we'd see USC pass Coastal Carolina in this scenario

I personally like this one - it gives everyone a chance in theory at least, but does anyone really believe that San Jose St is going to go on a run to beat Texas A&M, Ohio St, Alabama, and Clemson in 4 consecutive games (or that BYU would go on a run vs USC, Alabama, Ohio St, Clemson)? Biggest issue is that it's 4 additional games.