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Here we see Spivak defining Categories as comonoids in the monoidal category $(Poly, \circ, y)$, section 2.5. Comonoids in an endofunctor category are comonads. I would guess that if you add a monad structure to your comonoids, you still have a category, but you have extra structure. It would be interesting to know what extra stuff you are endowing the category with if you add monad structure.