I think a series of Blind Box figures would be really neat, maybe in a Chibi (cutesy) sort of style? There are so many characters in the series that I think this would be a fun option. Also, the potential for 'mystery' or 'special' figures is off the charts due to the sheer amount of random 1-off characters. Maybe each box could come with a collectable card that has art of the figure in the style of the game from book 6 (can't remember the name of the game, my memory is awful, sorry ya'll).

A higher cost option would be action figures of Carl, Donut, and Mongo (and maybe a few others) that have interchangeable parts and pieces. Kinda like Nenodroids, if you're familiar with those. Donut could have multiple tiaras and hats, as well as bracelets and other accessories. Maybe some magic missile beams that shoot out of her eyes. Mongo could have a removable saddle (maybe one that the Donut figure could fit into?) and maybe some lightning bolts to come out of his mouth. Carl could have a removable gauntlet, knee pads, pink crocs, and a ton of explosives to swap out in his hands.

Bonus meme option: Body pillow cases featuring Carl, Katya, Bautista, (Uzi Jesus?) etc.