I love isekai anime but there are lots: the old ‘technical’ isekais that just involve going to another world, then there are reincarnation isekais, videogame isekais (& subsub genre of otome villainesses isekais), alt-history is email, reverse isekais, death game isekais, … (Etc) there can be a ton of overlap where they sometimes borrow tropes and themes or are influenced by one another. Each has a history and a sort of genealogy, but there all isekai. The nuances, or even the very existence, of the subgenres can often be lost on casual viewers.

It’s sorta like that. Less well defined, and often relational. Borrowing sounds and ideas, or tones and themes. Blues rock is rock than uses elements of blues music, like like how blues jazz is to regular jazz. It’s an independent dimension from the main genre in that case, like punk rock… I wonder if punk jazz is a thing? Punk classical? Hmm…