This is something I've been thinking about for a while now, It'd be really cool but I doubt at the time of me making this post there's much interest for merch of my characters.
If I had to choose only one thing of merch I'd go with plushies, specially of my monster characters. I feel like they'd look the best in plush form. Stickers, buttons and pins are the second thing I'd choose, personally I don't really use pins/button as much but they're fun to collect and I've seen a lot of really pretty glittery ones before.
Shirts would also be really amazing too, I'm probably going to describe this really badly but I imagine it'd be a really stylized and would use certain color palettes. Like a kind of shirt where even if nobody gets the reference they'd still think it looks good.
Oh and figurines too, of both my monster and humanoid characters. Perhaps in the style of nendoroid figures? But a regular figure would be nice too.
Any type of merch would be pretty cool, even just prints.