I might consider going to AO3 if FF.net should ever shut down. After all, I've still got my fics which I've written 17 years ago plus backups. I can repost them whenever I please. I can also publish them here on my Hubzilla channel, maybe even nicely typeset in LaTeX. If they disappear, I can always have them come back.

That being said, I do ship. But I don't write 100% pure shipfics in which nothing else happens. I only really have one ship developing throughout my 'verse right now. And all ships in all my fics are het which might upset the typical AO3 user.

Speaking of which, I've only ever written and published pre-movie Chip 'n Dale Rescue Rangers fics. I'm a Rangerphile. Few people even know that the fandom exists. But those who do are very likely to have prejudices that'll want them not to have any of my kind on AO3. And indeed, if I look around the CDRR fics on AO3, there's one story by one author who's halfway known in the fandom, there's another story that was written by someone who clearly knows at least one fanfic classic, and only three CDRR fics were published on AO3 before 2018 at all. The rest was written by people who either don't know there's a fandom, or who do know and deliberately steer clear of it. FF.net has a lot of fics by writers with no connections to the fandom, too, but it's also one home of some classics by well-known authors.

Also, I think there are obvious differences between fanfics written by casual fans and fanfics written by people deeply involved in a fandom who not only know the canon inside-out, but who have discussed it with their own likes a lot, and who have also read a lot of fanfic from within that fandom. Fandoms tend to be echo chambers that produce and cultivate their own specific flavors of fanfic which may be hard to get for people outside the respective fandom, featuring very fandom-specific tropes and requiring detailed canon knowledge to such extents that even footnotes don't help.

I was way "down the rabbit hole" when I wrote my fics. I didn't read the most well-known classics at first, also so they wouldn't influence my own fics too obviously, but I guess the typical fanfic style had already rubbed off on me when I wrote my first multiple-chapter fic.

I'm not sure how welcome such fics from deep within a small but dedicated fandom would be on AO3.