Topaz
Here is an overview of Topaz--which as far as we know is not happening:
https://github.com/giesse/Project-SnowBall/blob/master/topaz/concepts.rmd
Interesting to look at is what he says about "promises" as a way to reconcile the convenience of synchronous I/O with the flexibility of asynchronous I/O. Yet he points out a weakness--in that you can't delay evaluation products if they drive conditional code.
I suspect what he suggests drives too far away from what Rebol is actually capable of under its paradigm, but putting it here for study.