I’ll disagree to the auto-fill disclaimer too. Simply because the authors should THINK about it, that it’s fanfiction (or not), and what the copyright position is in their work. (Not sure if the field enforces a minimum length, but it ought to… say, at least 16 characters?)
Second, I concur some distinction on the tags would be nice, whether the tag is considered more of a warning (ie scat CYA) vs an advertisement (ie exhibition tag due to persistent nudity). Custom tags would be nice, from an author’s POV. However, it’s also a bit frustrating on AO3 to know if I’ve got *EVERY* applicable tag for my story, so I clicked the “author chooses not to use tags” and proceeded to tag the heck out of it, along with “maybe I missed some?”.
Databases – combining them might be advantageous, along with a single (new) URL; though you’d have to preserve old URLs for a while, if not forever. If combined, then it’s resolve the category issue for crossovers, letting them show up under both, without having duplicates in the database.
Other improvements could also be had, but even switching to another software (commercial or otherwise) requires having people & time to execute it. Volunteering?