Not sure if my interest would represent any market, but I'm so into Jigokuraku right now. I know it's not an anime (yet) but it's a really good manga. Dungeon Meshi and Given is also a niche, I guess.
I love Ito Junji's work, but I'm not sure how people would like it as a merch lol. I'll second (third?) HxH, always a classic.
Hamefura is one that I see got a big fandom but no merch for yet. Promised Neverland is also good since the manga just recently finished.