I haven't played Monopoly for many years now. The last game was with three housemates at university. One housemate was winning and really enjoying his triumph over the rest of us. So I suggested to my other two housemates that we form a cooperative. We would let each other off rents on our properties. The standards rents would apply to anyone not in the coop. And you could leave the coop if you wanted. None of this is forbidden in the rules of Monopoly because it has little market regulation.