Hi,
I appreciate your project, Rei - but would like you to think about if this is really necessary: to start/develope a new one.
Don't know if you realized it, but Lukas did make a really good Lite-C header for creating GUI elements called LBGUI. And he continues to improve it - with a great support and eye on user feature requests. (thx!)
Anyway, an editor sounds great. But maybe you can join Lukas and continue with him his great work (he's german as well). Even though it's not dll grounded.
Greetings