Mick, you might be interested in the Sauerbraten game engine: it is a first-person shooter that features an ingame editor you tap E to switch into Editing mode , and the editor can function perfectly well online with multiple people working simultaneously. http://sauerbraten.org/
I'm of the camp that believes that only when you identify your concepts and vocalize them can you move beyond them. By stubbornly refusing to define game concepts through an informal consensus, we hamstring ourselves. Not only are we keeping ourselves from finding new ideas, but we're also ignoring the ...
