But who is supposed to decide if not you? I mean, even if the archive allowed original fic (and I hope it won't) it would still not be a place where you could put everything you ever created, because otherwise it would not be a fanwork archive at all anymore but a social webhosting space where people put personal archives of whatever and share tags and a search engine. You would still need to decide which of your things you could archive there and which not. To restrict archiving fic to things authors themselves see as "fanfic" is not any different.

Unless you really want something that stores everything for anyone or even just everything for anyone with one fannish token in their works, but that would mean storing all pictures (because the boundaries for pictures are not any clearer between fannish and original), to all knitting patterns (because gryffindor sweaters or knit Merlin puppets are fancraft and there is a sliding scale for knit works just like for fic), all clothing patterns and photos (because costume making is fannish, and most would have no trouble for people to upload their costume photo as fanart), and eventually once it allows vids that would mean all videos too?

I mean, there is nothing that makes original fic extra special or more complicated than other fanworks. I think it is important to keep in mind that the intention is a space for fanworks, even if boundaries are fuzzy in places. And I agree with copracat that considering all the variable definitions of "fandom" the easiest way to do that is to just depend on the judgment of the uploader whether they think what they upload is a fanwork or not.