I don't think there's much to debate, honestly, if you think there's the slightest possibility that my P&P or Shakespeare fanfic isn't Real Fanfic because of the age of the canon. I don't want to be harsh, but I find the idea of this both completely absurd and rather insulting. I don't think the law has any authority whatsoever as to determining whether a fan's work is truly fanfiction or not, only if it is potentially copyright infringing or not. When, for instance, Diane Duane (an Irish citizen and long-time fan) says that her Star Trek novels are fanfic that she was lucky enough to get a chance to write for money, this is not a legal matter. Other fans might feel differently than Duane about her novels and have an intrafandom discussion about it, but lawyers and judges do not have any special authority in that discussion.

And as I said, I wasn't making this post with any intention of providing a specific formal definition of fanfic, and I still have no real intention of doing so. I was only pointing out the many, many fanfics whose status as such has never been serious contested in their fandoms that are being overlooked when pro writers try to exert authority over what the rest of fandom considers fanfic.