Typically the teams outside of the top 4-5 aren't going to have much chance.
However, I think you need some protection for the G5 conferences, but not a guaranteed spot for every single one. Top 6 ranked conference champions plus 10 at large - no guarantee for P5. I would be OK with that. You can qualify two ways that way. Seeding based on ranking. Yeah, there is bias.
No way the P5 agree to that. It will be P5 plus 1 at best. I don't expect it will ever expand beyond 12 either, but I wish it was 16. At times, there are some very good teams in the 12-16 range and it allows more P5 to qualify.