I do like the idea of something about coming from the inside vs the outside, but I'm not sure how I would word that into a definition. Hmm.

Requiring an official seal of "approval" does automatically throw everything into fanfic where an official seal of approval isn't an option - that makes Shakespeare fanfic and Chaucer fanfic and the Les Miserables musical fanfic, and any fiction about historical people whose estates aren't actively managing their image fanfic. Which maybe it is - I generally include it there.

It does seem kind of weird to me to say that Shakespeare is fanfic but Splinter of the Mind's Eye isn't, though.