I really like this project, 'cause this is what I searched and hoped a long time for.
For your development I've some suggestions:
1. Blizzard compliant XML-Visual Designer (Try to load some .XML from the original Blizzard UI addons... it's really unattractive and confusing)
2. Like the one post before mine, a LUA script debugger or at least a LUA function&syntax checker...
3. XML Template support... customized Frames and so on... the things you really need =)
4. "implementation" of the Ace2GUI addon which is really great ;)
5. More control over the order of the files in the "compiled" .toc File...
That's it for the moment...
I will develop one addon and then hopefully have more Ideas =)
But really, it's great, great, great!