Not gonna lie, an art request board would be awesome. I'm starting up a Mutants & Masterminds campaign. Just had our first session last Wednesday and we had to resort to using a terrible whiteboard since Scriblink only allows 6 people at max. I've got a pretty decent sized group, so needless to say, that didn't work for us. It wasn't until after the session that I chanced upon Roll20 and saw that my prayers for an awesome, tailor-made virtual gaming tabletop were answered.
With that said, as we all know, the app is still growing and evolving. While in its current state, Roll20 is indeed leaps and bounds ahead of anything else out there, it's certainly not as user friendly as I'd like it to be, mainly in the art department.
An Art Request Board would probably be a fantastic idea simply because it'll help the community not only help itself, but also signal to the Roll20 dev team what content they're wanting to be added. Right now, some genres and time periods are a bit lacking and need to really be searched or simply found and added/uploaded manually. Fantasy I can tell is certainly covered and for good reason (DnD isn't the major powerhouse for no reason after all). I'd like to see more art assets for things like modern day cities being added. My Mutants & Masterminds campaign has plenty of supernatural and superpowered areas for my players to explore, but it's also got just as much normal stuff in it as well.
Anyways, long first post short, I think an Art Request Board would be a good idea. The more art assets Roll20 has to draw from its databases, the better, I would assume.