i'd say my #1 is vocaloid but i dont know if vocaloid is niche enough anymore. though i guess unpopular vocaloids is more niche. my favourite is iroha nekomura!
my second would probably be game/song novel adaptations
i usually play games for the story than gameplay so... novel adapts really appeal to me. you usually get more deeper insight on the world/inner monologue of characters you usually wont see! it's so cool to me to see a world you love just with extra detail. i got the ico novels recently i need to read :3
and song novels kinda ties back to my vocaloid interest. a lot of song novels are really amateur done but that's the appeal to me. someone made a song they loved but then wanted to add more onto it and you can really see the passion and love by the creator. i love when you can see how much a creator loved their work
my newest one im getting into though is drama cds. milgram, strange eden, clock over orquestra, boys be maid are some of the ones i'm been focusing on. do recommend me more ^^