The one subculture/community I haven't seen much on Mastodon is #fandom. I know ppl from fandom on here, but there hasn't been a significant enough presence that I could find "my people".

Mastodon would be a great home for fandom. You can have per-fandom instances, or have a fandom-specific instance (preferred). There's built-in functionality for spoilers and cw's & ability to set per-instance rules for interaction. The segmentation of identity would be useful as well.

Somebody do the thing!