So right now to do custom art for a campaign, we'd need to have it in the root data folder, correct? This can be a bit odd if someone wants to uninstall someone's campaign, as they'd have to hunt for all the extra art. Also, if someone wants to have alternative art for something, people would have to replace their files and possibly mess up other campaigns.
Perhaps campaign folders could have their own data folder. The game could look for files in that data folder first, and then go to the default folder if the file doesn't exist. That way any art is easily uninstalled, and alternative art wouldn't have to replace all other campaigns.