Harry Potter has 600K works on AO3 and 852K on fanfiction.net (with plenty of overlap, granted). That's #1 on Fanfiction.net, and in the top 5 for AO3. AO3 is #53 globally, Fanfiction.net is #572, and Wattpad is #323 by Semrush. Given all that, calling HP fanfiction niche is pretty funny.

And no, not a member of any cult, just someone who enjoyed a send up of Harry Potter.