For me, I would go with: - Typed javascript should be built-in in browsers. (Typescript) TypeScript is great, but all the configurations and transpiling is a pain.
Typescript should be compileable into WebAssembly bytecodes and that's it.
If you want it to be transparently compileable, like foo.ts to be sent as WA bytecodes then something like mod-typescript (on the fly typescript compiler) can be designed that will send (compiled and cached) bytecodes.