That last post got me thinking: isn't counteractuality an example of something that is ideally conceivable and yet metaphysically impossible?
On the standard understanding, p is metaphysically possible iff there is some possible world where p is true. But there is no possible world at which any world other than our own is really actual. (Just like there is no other country from which Wellington is not the capital of New Zealand. Once the index is fixed, the truth of an indexical statement is likewise fixed, no matter where you might "ask" it from. Our world is the only 'actual world' in the modal multiverse -- a static fact that's true everywhere you might ask of it, even on other possible worlds. See also my old post on modal indexicals.) So counteractuality is metaphysically impossible.
(It seems a straightforward consequence of this that there is no such thing as real possibility. The standard metaphysics of modality, as I understand it, thus commits us to narrow fatalism. This is a most unfortunate consequence. Perhaps I've just misunderstood the standard picture -- if so, I hope someone can reply and set me straight. Otherwise, I think, the standard picture is in serious need of revision. See the latter link for a somewhat radical alternative designed to avoid such problems. For now, though, I'm working with the standard picture.)
Nevertheless, it seems perfectly conceivable that some other possible world could have been actualized in place of ours. Indeed, I suggested in my last post that whenever we imagine, we imagine the fictional world as (counter-)actual. So we conceive of the impossible all the time. Unless there's something wrong with the analysis of possibility described above...
P.S. I've previously suggested a connection between conceivability and our explanatory practices, to the effect that, if Y is a conceivable alternative to some actual fact X, then we should want an explanation of why X is true rather than Y. Conceivability is the important notion, not metaphysical possibility. (Just as well, since the latter notion is so difficult to make sense of.) So, even if it's metaphysically necessary that our world is actual, it'd still be nice to have an explanation for why this world and not some other.