If -- and that's a big "if" -- Wayland's model is determined to truly be the successor to X, then it's really just a matter of adding a new backend to a toolkit that already has half a dozen.
If GTK was to get popular on Wayland then that means that unless new features are supported both in X and in Wayland then that means that GTK won't be able to benefit easily from improvements in either one.
Almost all interesting features -- including allowing apps to move between X servers like you just asked for, as well as networking -- are really toolkit problems and library problems. A modern toolkit's abstraction layer needs very little outside of a way of getting hold of an OpenGL context, getting input events, and low-level stuff like threading and I/O and basic audio. Current toolkits have bigger abstraction layers mostly just because they were designed in the day and age when the X drawing API or Windows' GDI API were actually considered useful.