Hello @matt.duffield
With pleasure, As you know better than me Monaco is an awesome editor with intellisense and highlighter so suppose I want to write C# editor with it. I am not sure it is possible however we have breakpoints and debug mode in VS Code but in online mode, I don’t know but it would be great if the custom element helps us to create breakpoints and debugging codes in the online editor.
something like this for python:
Online Python Debugger
You can add breakpoints and debug the codes online simple but useful 
If this technique possible (Enable breakpoint/debug mode), you can add it to your awesome FrondEndDeveloper tool too, especially for typescript.
I know in Ace Editor they have some functionalities.
Add/remove breakpoints in Ace editor by clicking the gutter space
C9 Breakpoints
In summary, A custom element with
– Adding breakpoints
– Debugging mode and watch variables