Sounds like a nice suggestion, but if you're going to be looking to make money using someone else's work, it sounds like you're going to have to put in the work to track down the authors and get their approval. I'm new here, but I'd hate to see roll20 get bogged down in asset licensing for 3'rd party assets and working less on roll20 itself. Also, you might want to make it an "official" suggestion and post in the suggestion forum if you can. Might get some more traction there if people really want this.