One neat thing I see this has is colorless async: https://fubark.github.io/cyber/#colorless-async. However this is a work in progress as their async isn't multithreaded yet.

Interesting, but I fail to see how it can work exactly. If there's a function awaiting something, how is the function called then? Will it look like calling any function, but it might not return immediately? But then there's also examples of calling aio.delay(1000) which does return immediately. I think my question is ultimately, what's the return type of a function containing an await?

Well I'm not the author so I can only guess the answer to your ultimate question: if a function contains an await and returns a string, my guess is that the return type would be "string".

After all, that's what "colorless" implies to me: it's possible to take a non-async function and make it async without having to change its callers. This suggests we don't change its return type.

This pages gives a good overview and motivation for Cyber: https://cyberscript.dev/