Actually, I think system agnostic settings would be awesome. People are always converting settings from one system to another. That conversion could be a lot easier without the baggage of an "official" system. If someone did the the world building and provided the stuff for adventures, I'd probably snatch that stuff up (assuming the setting interested me). All the character sheets need are a name, a token, and a bio. The Bio could include basic info on what are the characters strengths and weakness (just speak in normal terms) and I as the GM would be able to assign what I feel the appropriate values are to stats, etc. The great thing about Roll20 is all the character sheets have the same name field for the character and the Bio & Info field aren't even part of the character sheet, so it could all be set up automatically. I purchased the D&D Monster Manual for my Savage Worlds games, because it has great artwork and tokens, and when dragged onto the table top, I just use the Bio & Info tab to figure out how to state the monster for Savage Worlds. I've done the same thing for certain D&D modules...I purchase them not for the stats, necessarily, but for all the other prep work that goes into designing an adventure and setting it up. For traps and the such, just indicate how difficult the challenge is to overcome (easy, average, difficult, very difficult)...