As far as participating with other fandoms, I only frequent a Resident Evil forum from time to time. Used to be big with the Silent Hill series back in the day too but that series has practically died.
Same as Xenomrph. I'm a fan of plenty of stuff but don't participate in much fandom stuff for any of it. Closest would be The Inquisitorium, which is a group pertaining to artsy and hyper-gothic takes on the Warhammer 40k setting. Pretty specific, and it might classify more as a club.
MCU, Terminator, Die Hard, The Thing, 300, Blade Runner, Miami Vice, M.Night's Unbreakable Universe, Conan the Barbarian, LOTR, Mad Max. Star Trek OG. Tron. Basically anything but aliens with acid blood and other-worldly hunters from space.
In terms of actively engaging online with other fans, probably only really Stargate. I'm in a bunch of FB groups for other interests, so I can keep track of news and etc but I don't engage with other fans.
I'm a fan of a bunch of other things, but I don't know that I engage with many other "fandoms" than this one. I chat a fair amount about Transformers and Warhammer40k on other forums, but those forums aren't dedicated to only those topics.