Getting better repl tools, and language integration with the repl, are two of the main things I'm interested with in arc.
I've only gotten as far as 'src and 'ppr, but I was hoping to get to the point where arc automatically documented it's current code (i.e. the stuff actually running) and made it visible through both web and repl interfaces. In theory, 'src could be used to solve the repl -> editor -> file problem. If arc kept track of the original source files, and the current 'src associated with each of those functions, then it could possibly display the diff and even update the files if you wanted it to. Heck, why not give it a git interface, so that you can add and commit changes from the repl as well.
The reason I like arc is that the language is very easy to change, and I'm hoping to change it to make it the language that is most "aware" of itself, and its current source code.
Making a better repl is certainly something I'm interested in.