I plan on working on the in-game code editor next. I've tentatively decided to just throw caution to the wind and dynamically compile C# code. This, however, requires a good in-game text editor. Gwen has a great looking UI, but no multiline text box, and I will get on that as soon as they merge my giant list of changes with their project. Hopefully, I can end up a commiter over there too.
I'd like you to be able to program in world gen, block interaction, and game modes (trouble in block town, ftw), so I need to start thinking about how to make every block type you make, every texture you create, and every piece of code you write, fit into a nice modular packaging system that can be carried over neatly between servers.