This looks quite promising. What are some new features you'll be adding to it (that is, if you're planning to do more than a rewrite)?
Qt has great image manipulation support, so it'd be cool if mappers could resize, shift, and move around parts of the map. It would
not be an image editor, it would be more of a walkmask adjuster. So the mapper would create the basic shape in his drawing program, adjust everything in Garrison Builder so that gameplay felt right, and then go back to his drawing program to repair the damage Garrison Builder had done to the background during the adjustment process. I think this is important, because currently the tools don't encourage mappers to focus on gameplay.
Also, I want to redo control point support to be much more powerful. So the new editor would support that.
It'd be really, really cool if all the entities were loaded by the Builder from a zip file. When new stuff was added to the game, mappers could instantly get access by auto-updating the zip file. Even a non-coder developer could maintain the zip file, because it would just contain png images and text config files.
I am really looking forward to this 
Does this mean the compiling will be faster?
Yes, compiling should be instant. There's really no excuse for the current builder to compile so slowly. It's my fault for doing it in GML.