I wanted to say as well that I enjoyed your recent post on fictional vs. possible objects. You point out that being internally contradictory is not a necessary condition on being metaphysically impossible. This seems to me exactly right. Another way to make this point is to think about, for example, a necessarily existing unicorn. There is nothing internally contradictory about the idea that a unicorn might exist in every possible world. Yet such a being is surely impossible. Otherwise, if it were possible, then there would actually exist a necessarily existing unicorn. This follows by the modal reasoning we find in Plantinga's modal ontological argument and, in particular, the distinctive axiom of S5 modal logic. In order to avoid Gaunilo-style parodies of the modal ontological argument, we must deny that being internally contradictory is a necessary condition on being metaphysically impossible.
I accept John's example and his reasoning. Ain't agreement grand? We philosophers must enjoy it when it comes and while it lasts. And so we can add the necessarily existent unicorn and his colleagues to the list of metaphysical impossibilia whose impossibility does not derive from internal contradictoriness along with internally consistent fictional objects such as Hamlet. Are there any other categories of metaphysical impossibilia?
Many scholastics would add extramental universals and privations to the list of metaphysical impossibilia despite their lack of internal contradictoriness. Thus humanity cannot exist outside the mind. Nor can blindness.