Shameless plug: I built something similar as a part of my workflow platform for JVM (for Java and Clojure):
The main focus of my tool is enterprise integration and data processing, but I still love to play and experiment with the ideas of developing code directly in browser and how to visualize its execution flow.
So I built a live coding / visualization also as a part of my tool.
This becomes non-trivial if you apply it to languages other then js (so browser integration becomes a bit harder).
I also think the main challenge here is the server-side part of things, especially when you make such solution distributed - i.e. how you achieve high availability and assure everything gets executed (at least) once even in case of failover, how you cater for multitenancy while achieving sufficient security/isolation between users etc.