The tricky part is the "getting people to work together" -- and getting enough others interested... just from what I've seen on forums mostly people prefer to create their own complete images. The closest thing to what you suggest has been with the old "fodder" threads we used to have here. These happened to revolve around members contributed photos of themselves and then others modified them into funny comical scenes, etc. This present thread sort of uses some of that idea too, in that you first investigate the starfield and planet tutes to get the hang of making some and then you create your own type of astronomical imagery from there. Contest threads are similar in that they often present a starting image and then you have to modify it to a complete and new one yourself.
As we are all at a distance online we can't actually work with anything other than what's posted mutually in a thread and shared for a project...but from there the idea that comes to mind from your suggestion is working on a single image project where everyone takes the image as has been added to or modified by the other users and adds their own portions/modifications to it. For example say someone started the basic black 600 x 800 square with some of a starry field, then another person modifies it more, then others add more, eventually add planets perhaps, etc. This would work better if we had a huge space like a wall for a mural to work on...so since it would be limited by the size here, the project thread would probably complete very quickly and to keep such a thing going you'd have to start many new ones. That's probably why it hasn't already been tried here.
Give it a try, start a new thread for it, if you've got some other friends who want to do this too, so it will keep going...if you do though, be sure to make it known to those who post that their images that they will be used by others so no one gets upset about that. I can't speak for any others regarding use of the images already posted...generally it's not what people want to see happening without their being asked.