ImGui is reliable and Zig makes it trivial to use.

Fair enough. I was referring to the GUI framework space in general, not specifically for Zig

In what general direction would the innovation be?

I'm not sure if it's actually needed - a lot of things have been tried and the paradigms that persisted you can choose between but it seems to be a matter of taste. I often feel that the real issue is the lack of maturity and stability. I rather want a complete UI toolkit including learning resources (not reference docs!), no matter how boring, instead of super clever implementation that is never in a usable state.