There is a growing and unseen need for a Mono-compliant development platform. This would be something like Eclipse, but much lighter in base. This would allow people to extend it very easily in pretty much any language and a lot of different targets could be found. Extensability needs to go very far, too. Syntax highlighting and code-folding rules is not enough. If I want to define that all lists and tuples be displayed an element-a-line and no more than 5 lines used (with an embedded scrollbar to go through it) then let me do that. If I want dictionary literals to be displayed as tables, then so be that. If I want to very the if and else blocks side-by-side in color-coded boxes, then let me do that, too.
Just a thought.
No, this should not use Gtk+, because that isn't productive (.Net has a GUI API, already!).