I like your idea and I had a similar proposal, where Fighters who wants to fight each other just sign to fight regardless of a fixed weight. If they already signed the contract, I'm sure they don't care if the opponent came in heavy or skinny as possible, because they've already made that calculation when they decided to fight this person.
Like if Mousasi and Rockhold decided to fight each other, both won't really care if both were walking in heavy or light. If the difference seems too much where creates a disadvantage to one side, they have option not to sign the contract, and don't fight. But my point, usually when Fighters wants to fight each other, they don't care about what weight they'll be fighting. They want to fight that particular opponent. I doubt Mighty Mouse will challenge Jon Jones you know?
They have to weigh-in on the day of signing and weigh in as they enter the cage. (for ranking purpose?)
There could be a freak show scene but that's where commission comes into play like if it's 30 pound difference during the day of signing, don't allow that shit.
Rankings, Bonuses, Champs could be sorted out after based on the success at which weight they took the win at (weighing before entering the cage).