I would be so, so happy to see a UI framework that runs (well) on Windows, Mac, Linux, Android, and iOS. My druthers would be for it to have bondings to a JVM language (Kotlin is my happy place these days), but I would be okay writing in any similarly productive language.
Flutter is trying to get there, but it's not a viable solution on the desktop yet, and somehow doesn't even have a decent HTML widget. I do like Dart, however; it's similar enough to Kotlin/Swift that I'm comfortable writing it, even if I have to look up exact syntax now and then.
Electron wants to solve this problem, but we all know the arguments about its bloated runtime and egg-cooking power usage.
I suppose Progressive Web Apps and WebAssembly have the potential to solve this, too.