I'm not sure why nobody is talking about TypeScript? If browsers were to natively support it, that would be a game changer. We could phase out quirky features of JS and mark them as obsolete in IDEs. More use of the Map class for example would avoid some quirks of using objects.

TypeScript is designed in such a way that type annotations can't influence the runtime behavior. This is unlike e.g. Haskell, where types can drive code generation. Happy to elaborate more if necessary.

This means that transpiling TypeScript to JavaScript consists merely of stripping the type annotations + transpiling some of the syntactic features that might not be supported by the target JS version.

The "Type Annotations" proposal (https://github.com/tc39/proposal-type-annotations), IIRC championed partly by the TS team, will allow type annotations with almost-TS-like-syntax in plain JavaScript, which will make the "stripping type annotations" step unnecessary. You would still use the TS tooling to actually type-check the code, though.

Which exact part of TS do you want to be built into browsers?