Mixed reality would be perfect to fit the games to the space you are in, a game like superhot could be set in an environment that reproduces your room, with enemies proceduraly generated!
But for a more short-term vision, mixed reality could be used to better set up the Guardian! For example certain objects, like a table or a bed, only limit my "whole body" movement, but i should be able to move my hands above them! With the current guardian setup, this is not possible!