If the UFC is willing to put you on as a main event and you win, why not give out a % of the gate? Start super small and eventually it goes up a bit over time if you keep main evening, become champ etc. Would that work?
Why would they need to do this? Some fighters get a % of PPV buys. Some fighters. This is what contracts are for. You agree to fight for X, then you fight for X. Easy Peasy. Why must be complicate things?
Don't main eventer's usually get ppv points? Honest question, I don't know if it's just champs, or certain big draws, or if it's every main event.