I don't use any zero calorie sweeteners, so I can't comment on how those might work in this recipe, but keep in mind that the granulated sugar is what gives these brownies their classic "fudgy" texture. If you want to use maple syrup or honey, these brownies will probably be much more wet and fragile, so they might lose that classic brownies texture, and you won't need nearly as much, since they are sweeter than coconut sugar. (I'd start with 1/2 cup maple or 1/3 cup honey.)