Yeah ImpactJS has an integrated editor. It only supports tile based maps so it's the exact opposite of C2.
I looked at impactJS a while ago, seriously considering plonking down the cash for it. I looked at avideo going over how the editor works, and I got to say it looks really, really good. I would not mind some sort of crosss compatibility with it and c2 (kind of like the GM rooms thing) or just using it as a source of inspiration and innovation.
I've been wondering, from time to time people ask/talk about scripting in C2, and in general that's what the SDK is for. Couldn't you, in theory (I don't really know) make a plugin that lets you execute code from events? Sort of like what Pythin was in CC (though I never really tampered with that, so I might be a bit off-base there). Just a thought.