It's an interesting idea: to have both types and rich configurable pre-processor like babel. Is it viable or would it be easier to use flowtype for types and babel transformers for everything else?
IIRC, flow doesn't/didn't work on windows, which is a non-starter for a lot of users... not sure where this stands now, last looked at it when using flow types to be able to use angular 2 without typescript, as I mentioned, I didn't see the point as I still needed bits from babel.
I am actually using typescript only for types and then passing the code (even with JSX) to babel to do all the cool stuff we know it can do. So async / await is no problemos.
But flow adds type support (same as typescript), and can also be used with babel... flow only does one thing, so better integration. Typescript doesn't offer much beyond types (and flow does that) over babel.. and as you say, you're using babel anyway.