Hmm, with layer-shell, you can run some parts from other DEs under a given wayland compositor. I'm not sure what parts support the protocol, but I'm fairly sure you can run krunner, and probably the KDE status bar under sway. I think you can also run xfce's equivalents.

I don't see why all of that couldn't be integrated in nice, modular packages and sets of configuration. Some say that wayland made everything more monolithic. If anything, I think it helped make everything more modular by forcing to specify the interfaces into wayland protocols.

Now, of course some DE prefer using private interfaces instead of proper protocols...