Mine are very strange to say the least because they probably only exist in one fandom and even there only in its inner core: self-introduction fics and mass-insert fics.
They're a bit hard to explain, much less justify. They only exist at the Acorn Cafe, the central forum of the (pre-2022 movie) Chip 'n Dale Rescue Rangers fandom, and they imagine it as an actual café with all kinds of extra amenities imaginable or unimaginable that's frequented by the Rangers, their friends/significant others/family and the fursonas of at least some of the forum users.
Both are subgenres of self-inserts. The difference between these and more common self-inserts is that the self-insert is absolutely blatant and obvious because these stories only work if it's clear that they're self-inserts. The self-insert OCs are practically never humans and always anthropomorphic animals to make it easier for them to interact with the Rangers. Thus, they're only your usual wolves, foxes, dragons etc. if the author has had this fursona before becoming a Rangerphile/discovering the Cafe. More often, we're talking about mice, rats, chipmunks and squirrels and self-insert OCs purpose-made for the Cafe.
Self-introduction fics are stories which tell how a new user's fursona arrives at the Cafe for the first time, usually meeting the Rangers there. Not rarely, self-introductions are also mass-inserts.
Mass-insert fics, in turn, aren't always self-introductions. They are when someone not only includes their own fursona in a story, but also those of other Cafe users. Care is always taken to write them in-character, also because there have usually been enough "templates" from which to see what's in-character for them.
Mass-inserts can be a lot. They could be a self-introductions that includes Dr. Indy, the Cafe admin in real life and a chipmunk in the story, having tap duty when the new first-time visitor arrives. There could be other Cafe patrons sitting at tables or maybe chatting up the new visitor before the Rangers show up. They could also be casework, either along with the Rangers or taking the place of the Rangers who happen to be on another case themselves or actually rescuing the Rangers from some villains. This could delve more or less deeply into Mary Sue territory, but that's okay because just about everyone else is a Mary Sue, too.
A specific sub-subgenre of the mass-insert is what's called a "round robin" at the Cafe. It isn't really a round robin; authors aren't fixed, their order isn't fixed either, and instead of planning the story to be written ahead, everyone roleplays and makes up stuff on the fly. The Mary Sue factor may be even higher here to the point at which many of us got themselves spaceships based on various franchises.
Technically on the edge of the mass-insert subgenre, too, were the annual Golden Acorn Awards ceremonies. Not only did the fandom have its own awards in quite a lot of categories, but instead of just nominating, voting and then giving away awards, they were turned into full-blown written ceremonies, what with the writing talent amassed at the Cafe.
These ceremonies took place in various places, the first ones at the Radio City Music Hall, the others at annually changing places nominated by the Cafe patrons and then voted. Most are real locations which once included a cruise aboard the Disney Fantasy, but I've once managed to get the Muppet Theater voted, not spectactular per se, but it promised even wackier shenanigans than all the previous locations because we could cross the whole thing over with the Muppets. The promise was kept.
Most of the writing was done by the Cafe patrons. Dr. Indy organised the whole thing, he'd put everything together and post it in the end, and he wrote a kind of parallel storyline in which Fat Cat and his gang always try to steal the Golden Acorn Awards.
The presentations were written by other patrons who picked one or multiple ones. Normally, they didn't use their own fursonas as presenters. Instead, these were either canonical characters (the Rangerphiles love one-shots, so there are many to choose from) or their own fanfic OCs or their own roleplay/round-robin OCs or sometimes crossover characters. Sometimes they did use their fursonas which became kind of awkward when they had to give an award to themselves. The presentations sometimes entered mass-insert territory when the writer had someone else's fursona throw in a commentary or something from within the audience.
The acceptance speeches were, of course, written by those who won an award, but they had to keep it secret until they had officially received the award. It was then up to Dr. Indy to graft the acceptance speeches into the presentations.
The ceremonies themselves were led in by the "Live from the Red Carpet" segment which everyone wrote and posted themselves, and in which everyone described how, with what kind of vehicle and in which attire they arrived at the location. It started a few hours before the ceremonies.
The actual ceremonies were posted by Dr. Indy, starting with an opening and a first Fat Cat segment. The presentations came in three blocks with breaks in-between (this went for hours!) in which Fat Cat appeared again. They ended with Fat Cat's defeat and a closing and led into a party.
Afterwards, there was always a round-robin party which, in real-life time, could go on for days, weeks or months even, depending on how wacky it went and what occurred in its course.
I've actually decided to follow the ceremonies in real-time a few times. This wasn't easy for me because they started at 8 pm EST/5 pm PST which was 2 am for me, and then they went on for three full hours. On two occasions, I pulled an all-nighter and stayed for the party. Otherwise, I went to bed around 5 am and came back to the party going on at full bore. It felt worth it. Later, however, I read the ceremonies the next morning.