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I was planning to randomly assign tasks to anyone. I only know of one interactive thing with shared access, and it's just a drawing tool, which isn't very cool.
A more complex option is for everyone to send in random things, and Reven combines them into a cohesive image, but that would be more of a fun project for him than for everyone.
We could do the opposite. Everyone sends in a few elements, Reven mixes them up, sends a few other random elements to each participant, the participant creates a picture from them, we post it, and we all laugh, but in that case, you'd definitely need to be good at Photoshop or at least have some decent Paint skills.