Heterogenous graphics card configurations can be supported on Wayland (e.g. Sway 1.0 supports them). GPU hot plugging could be supported by compositors as long

Heterogenous graphics card configurations can be supported on Wayland (e.g. Sway 1.0 supports them). GPU hot plugging could be supported by compositors as long as you don't need the compositor to change the GPU it's rendering on (it's just not clear how this looks like API-wise -- and it's very difficult to obtain hardware setups that support those). Clients could then be restarted to use a different GPU. Wayland is not not limiting factor here.