As you said, I'm for a sliding scale. Any school can be elite, whether you are in the MAC or the SEC. If Penn State goes 8-5, 10-3, 9-4, 7-6 should they still be a top team? NO.
I'm also for logic in firing pertaining to where the program is at the beginning of each coach's tenure. For the sake of argument, lets say there are five levels of schools:
If a coach takes over a team in the "pack," where the majority of average teams are, then they should have a certain winning % and rankings expectation to maintain their job. If they drop below that for 3ish seasons, they are relegated to "Easy Wins." If a team drops to the level below the one they were at when the coach took over, then that should result in firing.
Put another way: The wonderful, masterful, all knowing jibe takes over an "Easy Win" team. They were 5-8 the season before he arrived, and had dwelled in the 5-7 win range for the past 10 seasons. Well, jibe kicks butt in recruiting, and his first season they go 8-5. Then, they follow that up with 9-4 and 11-2 seasons, with Level 3 bowls. At this point, jibe and his team would be bumped up to Pack status. Then, a season later, he has a great run, finishes the Regular season at 12-1, and makes the Orange Bowl. He's bumped up to a contender.
Well, now that he's a contender, he's ready to roll, but, he can't devote as much time to the team. They begin slipping, going 9-4, 8-5, then 5-8. They drop back to pack status. The next season is disastorous, resulting in a mere 2 wins. They go back to Easy Win status.
This is where my idea kind of breaks up; now that he's back to Easy win, he should be a) fired, for not improving the team, or b) he should remain until they drop into cellar dweller. I haven't thought through that part yet.
Anyhoo, This would allow for leniency for coaches who built their teams out of nothing, and be rigid for those who take over elites. One slipup, you're done.