I was thinking the same thing! A book, movie, TV show, etc. Give a few choices and people come up with their own interpretation of the selected one.
This is kind of poaching off Lucy Samuel's book club cookies, but I actually thought about it prior to seeing any of hers. I was thinking about a challenge that is inspired by a book, song, poem .... Could get the creative juices flowing.
But I also like your idea of the collaborative inspiration board.