In a dream situation, I would absolutely love LEGO merchandise. There could be little sets like Roara and Cane with the quad, or bigger sets that included a whole spaceship. I really like that LEGO can be both collected and played with, so that makes them really versatile merch.
Also, I would totally like to do some unusual merch like a make-up line or a tabletop game or something like that. I like usable merchandise, so clothes would also be great. Though, instead of simple print-shirts I'd like to design clothes similar to the characters on my comic or something like that.