Python

Yeah, a more modern take on mutexes (as boxes for values) and support for structured concurrency would be fabulous. Those are somewhat orthogonal to the goal of being able to write straight-line code without function colouring. Long ago, eventlet provided something like this [although it had smoe subtle and deep bugs that took a long time to track down].

Potentially, a major stumbling-block would be providing machinery so that interleaved FFI & python calls can cooperate (if that's determined as being in-scope, which it ultimately should be).