I learned while designing for animation with toys in mind, that the toymakers are geniuses. They can make any design work- even bad ones. But if you design like you were a toymaker; keeping in mind materials, joints (points of articulation,) scale, the parameters of detail, and what toy sellers look for that might appeal to kids, the closer the toy will be to your design. In other words, "if you design like a toymaker, the toy will look more like your design."
Of course, sometimes you get shitty toymakers.