I think an option could be GPU-accelerated drawing. It won't look native, but one can create a UI that performs well on just about any platform. Vulkan greatly improves the portability of such applications (compared to the OpenGL vs. OpenGL ES mess), and Skia has a nice set of primitives on which one could build.

It's also worth pointing out that creating a cross-platform API is better for new players than for entrenched ones.