Start with JavaScript. The basic syntax is delightfully direct, it has massive adoption already, the ecosystem is large, the runtimes are getting better all the time, compilation is here with WASM,
Now remove the weird parts (e.g., too much flexibility to redefine things, too much use of the global scope, too much weirdness with numbers, etc.), and add:
- Types (including sum/product types, Result<T>, Maybe<T>, decimals, etc.)
- More functional features (everything-is-an-expression, pattern matching, currying, etc)
- A comprehensive standard library.
Already this starts to yield a language that has the best chance to be what a lot want.
The other major advance in developer tools that I'm wanting to see is revamping HTML to have proper sophisticated controls built-in, controls that can be easily styled with inline CSS. This will reduce the amount of JS needed on the client.
These two things will yield a massive advance in programming productivity, at least as far as web-related development is concerned, IMO