This looks fun! Would be great to have something that catches runtime errors and highlights them in the source code. Then, of course, code completion etc to make it easy to find stuff.

I started working on a bit similar project some years ago (I unfortunately abandonded it...) but I got one thing right: the ability to highlight the erroneous line. See http://write-a-game.herokuapp.com/ and try to call a non-existent function.