I don't know about that. I think it would be best to make something specific to the Atomic, that is integrated into the editor like tools in Unreal or Source. My rationale is having it that way, even if it would make it longer to develop, would allow for tighter integration with the engine than any external tool ever will be able too, for example having all your textures and materials ready to texture your level with.
I wish I could actually make the thing instead of just talking about it, but I was never good with procedural mesh building.
//edit: Also I think it would be great to allow then make some sort of WAD-like system to go along with it so Atomic users, should they choose it, would be able to easily make moddable games without jumping through the hoops to do that like in other engines.