I'm planning out a WYSIWYG level editor for my 3D racing game, but will start with some more straightforward features like loading and moving models, before I get into more game specific things like building tracks and checkpoints. Having said that, as I'm also working on the graphics code that the game will use, there will inevitably be some level which it will have to respond to a WYSIWYG editor.
How "close" to the game code should the editor code be? Would you make it part of the game project, or in a different project that is deployed as a standalone program? I've tried using WinForms with XNA, but the combination didn't feel right to me. My plan is to start out with a simple GUI rendered alongside the game's graphics, like a toolbox for basic selection and creation functions. Nothing too big or complex like the editors for popular, commercial engines. I also found this, but I've yet to use it with my game...could be a timesaver if it's easier to fit it within my pipeline than making my own, though.