In the works for a little over a year. If there’s not much to show, that’s a good thing, because a lot of time was spent getting everything right instead of rushing to get something on the screen. (In case you couldn’t tell by the cornflower blue) 
Below is the UI editor, this will eventually be a program to be installed on the dev’s computer as part of my framework. If anyone has used Windows forms or Qt, to the right is a property editor window. To the left is the form being worked on. It’s obviously not done, but it shows the flexible modularity of the Widgets.
Multiple window support is working, and is as simple as it is in Qt or Windows Forms. (Or at least it was before I deleted that part of the code, but at least we know I know how to do it :P) Through some added template shenanigans, I should be able to make it as simple as right-clicking and adding a new MonoGame Window to your project, with which you can then double click and edit with the installed UI editor. Everything is designed to be similar to working with Visual Forms, from dragging and dropping controls to Events and EventHandlers.
I’ll make it open source when I either get bored of it, have a usable product, or someone shows interest in seeing the code. There’s a lot of work to be down still. For now, I just leave this gif.
I guess my project is very similar to the one here, though it accomplishes something very different. This seems like a hook into Windows forms while mine is a reimplementation.
http://community.monogame.net/t/monogame-forms-create-your-editor-environment