It's not about policing the creators (that doesn't seem, to me, to be what OTW or AO3, or Fanlore etc, is about at all) but asking the user uploading a work to decide this: is your work fannish or is it original? If you consider it to be original and in no way fannish, if you can't attach a fandom tag to it, then it's not for AO3.

It's easy to say that the decision is entirely in the hands of the creator. But I think you have to remember that the creator doesn't create in a vacuum. Fandom especially is all about community, and by having this very discussion we're taking part in creating a community view about what fits under a tent and what doesn't. I know that I am having second thoughts on labeling my own fic based on the ways in which this discussion is going.

So whatever rules AO3 adopts are definitely going to alter the discourse around this issue. I don't think that's avoidable.