We considered Flow initially as well - but aside from its built-in maybe types (we use Monapt for this!), it's a subset of TypeScript feature-wise that doesn't iterate as quickly.
Ah! I think I saw this awhile back. I didn't refer to it too much when working on Monapt since it seems to serve a more abstract purpose, and since Monapt's original purpose was to emulate Scala syntax.
let, const, decorators, string interpolation, other ES6/7 goodies.
Also a few other compelling things like abstract classes and community type definitions.