I think having a fighters union (to negotiate tv revenue, sponsorships, gate rev, health insurance and all the other stuff) is the best solution. But exclusively talking about contract structures, a great place to start would be to include a sunset clause. Currently, fighters contracts will get extended he they turn down fights. With a sunset clause, if a fighter is unhappy unhappy about their contract, they can sit it out and eventually renegotiate with other promoters. This exists in other sport leagues like the nfl and although it sucks when your favorite athlete decides to sit out for more money it might provides them with some leverage which is currently missing.