I just stumbled onto something called The Framework. It’s effectively an RPG sub-system (or perhaps meta-system) that can be wrapped around just about any traditional RPG, which allows for a sort of fluid, pseudo-GM-less way of playing. It allows players to take turns as the GM and as PCs, or even as supporting players if they have neither role during a particular scene. It would take a group with a high level of comfort with both each other and with the system being used to pull it off, but it might make for an intersting game. I shall have to keep it in mind.