A successful 'sequel' or 'homage' or intertextual reference will tend to be a reference that points towards its source, but also stands on its own. My feeling is that fanfic doesn't stand on its own, nor do most continuations of the universes of dead novelists.

This is a good point (although I might categorize the "sequel" with fanfiction, rather than with "homage"). People can and do read fanfic without being familiar with the original source, but they aren't usually the author's imagined readers.

I think that my own attitudes toward literary fanfic depend on my attitudes to the original: I'd like to see fanfiction based on the Aubrey and Maturin novels because I think that O'Brian made some poor authorial decisions. In this case I see fanfiction as (potentially) another way of talking about the original, which is what media fanfiction sometimes seems to be--expansion rather than (only) substitution, perhaps.