There are some stories I love passionately, and for which I will never write fanfic (unless perhaps for Yuletide), because they strike me as things complete in and of themselves, and any additions or changes would be like... I dunno, attempting to refacet a diamond or scribbling over the Mona Lisa. It can be done, but there's so little need and the required care and caution are prohibitive.

Whereas a lovely patterned rug that never had its edges neatly finished and tied off, but is trailing strings and threads every which way... well, that's a lot easier to work with. :-)