Must-have features for a compile-to-WASM native/web UI language?

Responsiveness support (as in ability to resize to different screen sizes and orientations in real time) at least as good as CSS. This is traditionally a limit of Canvas based UIs that have to use JS to manually reflow everything based on viewport dimensions. It's nowhere as nice as the CSS built-ins (flexbox, media queries, grid, etc.). (Not sure if that's what you meant by compile to WASM?)

I'd want a developer experience similar to MUI (not necessarily the styling, but the drop-in functionality and extensibility).

But then again I'm probably not your target audience either. As a frontend web dev I don't view my UI language as a significant barrier. React is fine. Other frameworks are fine. And things are reasonably performant these days. It's not declaring the UI that's hard, it's translating business requirements into synchronized client and server state. I'd rather have something that's easier to use than rtk-query than some UI wrapper written in WASM just cuz it's shiny.

Maybe...

  Multithreading (async / await?)
  Static types
  Generics (nullable)
  Controls / Widgets / Components which probably means classes and OOP (see flutter and web components)
  Makros
  Message Senders / Receivers instead of raw properties (smalltalk, Objective-C)
  Package manager
  Good tooling

callbacks/lambdas/method references seem like table stakes coming from the modern reactive web frameworks, but if you're doing Immediate mode or a more hybrid approach there might be more wiggle room.

It does seem like the big three (React, Vue, etc) emphasize declarative as best practice