I haven't been working much on this recently, mainly because of lazyness + classes + I finally got back to an older project.

I'm down to take contributions that improve the editor's usability, there's a lot of stuff I haven't remotely gotten around to yet (like selecting multiple things, copy-pasting them, editing a door or key without having to remove them and place a new one, editor undo). And once I add different backgrounds and tiles I'd also be down to accept cool contributions there. 

But as for game mechanics, I'm not very open to adding too many, unless it's something that seems like it'd have a lot of potential, or, unless it's some idea I had myself and thus am biased for :P. In principle, I'd rather have things that can't be achieved with the base mechanics, be hopefully achieved with complicated contraptions. And then only add a mechanic if it either simplifies a whole bunch of different and unrelated contraptions, or if it adds new capabilities in a way that makes sense and is hopefully simple to understand. For colors in particular, I think new ones aren't very appealing, since there's already so many, they'd have to be really special to be a good addition.

But who knows, maybe I'll change my mind on this stuff further into the project.

That being said, if you're really excited about an idea, you can always just add it and tell people to download your version of the editor to play your levels.