As long as an agreement has been reached by both parties, it would be nice to have the option to mark an item as a derivative of someone else's work so they get a share of the revenue as well. It would be in effect a type of licensing agreement. It's a simple idea but I bet a lot of people here would be happy to share their models more freely if they could retain copyright and a share of revenue.
This would be a great idea for full color sandstone items. That way people can color things the way they would like them to be colored.....or personalize something without needing any 3d modeling knowledge.