>What are certain types of doujin porn that are not commonly recognized as a category but should be?

I would say "shitting your soul/personality out as a piece of jelly", but that's usually tagged as such - "personality excretion".

I guess a niche is shotas crossdressing and being pampered by their older sisters. You'd be surprised how little of that there is. Pampered, yes. Crossdressing, yes. But combined? Not many. （ ´,_ゝ`）