Going to 16 teams could be tough.
Without the playoff,the four teams would still be in a bowl.
So the 4 team format adds 1 game beyond that,for the 2 finalists.
Go to 8 teams and its now 1 more game to get to the semis.Another for the finals.
Go to 16 teams and it goes up again.
16 teams also stretches out the schedule another week.
Either you start the first round right after the conference tournaments,or you're well into January for the Championship.
Personally,I think an 8 team format could work.
Go with P5 Conference Champions,1 P5 "wild card" and 2 G5 teams.
Based on economics,if you go to 8 teams,I would think the first round games would be at the higher seeded team's home field.
Have the first round games in December.
Then use the bowls like they do now.
This format also addresses the issue of "rust".
WFAN spends a lot of air time on this every year.
The consensus of pretty much everyone at the station,and fans that call in,is that they should go to 8 teams "sooner then later".