Harry Potter was my next big fandom. I had read and reread the first three books until their spines were warped. When Goblet of Fire came out, I got curious and was immediately traumatized.
Back then, there was kind of a weird divide between slash (as it was called when applied to western media) and yaoi fans. This is probably just my weird memory, but slash typically was more explicit than yaoi. But that might also be because I immediately sought out the smut.
The post-Goblet of Fire fandom was wild (as it was after every single book release). There was enough fic for me to read and never run out. I actually think Harry Potter was the reason I took my first looks at Fanfiction.net (which was huge and terrifying and hard to navigate for me who was used to single page archives or author pages).
As with Gundam Wing, the majority of the sites I used to read on are gone, but unlike Gundam Wing, Harry Potter is a fandom I still poke my head in and occasionally read in. Granted, I haven't touched Fanfiction.net in a hot minute because I'm usually on my phone and it doesn't block ads like my browser does.
No other fandom has grown and evolved with me like Harry Potter has. I mean, it's pretty much guaranteed that I will find something to read that I will come back to time and time again. The crossovers were wild too.
But we're talking about preteen me, not teen to twenties to current me.
I am 80% sure it was a HP fanfic site that not only got me caught, but got me banned from the internet for about a year. Probably the Potter Slash Archive or something weird like that. As I was saying before slash archives tended to be more explicit than yaoi archives and tended to have a lot of fanart just lying around. (which would be how I got caught).
I remember some really... out there fic. But there was some good stuff too. I'd actually love it if I could find any of the fic I read as a preteen, but uh, a lot of it would probably be cliche now and fall flat, even though they probably did it first. Also: veela fic, inheritance fic, Harry being adopted by x fic. The HP fandom introduced me to a lot of things (and I was very fortunate to be a lurker who never met any drama).
Also, unlike Gundam Wing, I actually have some recs to share when it comes to Harry Potter! A few of the fics I really liked (Secrets by Vorabiza, Hogwarts Apprentices: the Gentry Green by... Lisa?, Spellmaster by babychan) were on sites that no longer exist or were deleted by the author. Granted, it's been long enough that I can't remember what I liked and why, but I do have some bookmarks.
Harry Potter and the Natural 20. D&D based alternate universe fic in which someone's D&D character is dropped into the HP universe.
Trading Spaces. An old favorite, it's hilarious.
Elemental Genesis This and its sequel are very good.