Agreed, the art is going to be in the prep here. Typically when we would do workshops like this at frog, or in my role now, we would frequently have a pre session where we would brainstorm ideas and come to the actual workshop with a bunch of pre thoughts that could be easily spiced in. I think a mega rough mockup is OK (as long as it is popsicle sticks and bubble gum level), but the In Vision part will not be collaborative. Could easily be done with a paper proto.