I agree with the people pointing out that stickers and pins are likely to be what most of your player base can afford. That said, I would prefer plushies, and would consider spending stupid amounts of money if you offered one designed after the runty turnipling (or better yet, its evolved recolor).

Also seconding everyone who said they don't want character merchandise. I have no interest in buying anything themed after the NPCs, no matter how much I like them.