Honestly, I've found the most engaging stuff comes from leaning into the super specific things your fandom cares about, not trying to mimic popular BookTok trends. My corner is classic pulp sci-fi, which is basically dead on mainstream TikTok. Instead of doing the 'spine tingles' trend, I'll film a super close-up of my battered copy of 'The Rediscovery of Man', focusing on a single weird paragraph about a sunstone, and talk in a rushed whisper about why that concept rewired my brain. The comments are always from like five people, but they're the RIGHT five people, and they'll drop recommendations for even more obscure stuff.
One thing that really works is deep-dive comparisons. Not 'enemies to lovers vs. friends to lovers', but comparing the narrative structure in two different translations of the same niche novel, or how a specific monster design evolved across an author's forty-year career. It feels like academic shitposting, and the fans eat it up because it's content made for them, by someone who clearly cares as much as they do.
Also, don't be afraid to be critical! Fandom spaces can get echo-chambery. Making a quick video titled 'Three things the 'Viriconium' series does brilliantly, and one thing that nearly made me DNF' gets way more discussion than pure praise. It shows you're engaging with the work, not just worshipping it. That authenticity builds a tighter, smarter little community.