They're different in degree rather than kind, IMO. R&G centers itself on minor characters; so do some fanfics. That doesn't make them substantially different from the fics that center on the main characters. Stoppard does a more complex job with his story than most fanficcers do, using it as the launching-point for some kind of weird metaphysical thoughts, but I still think he's doing the same thing. It's just that what he's adapting is in the public domain.