A "non-prime world" is an abnormal world in modal logic where a disjunction can be true even if none of its disjuncts hold true. So they can be seen as a sort of "impossible" world. But this impossibility is not necessarily that of a direct threat of contradiction or explosion, it seems, like it wouldn't metaphysically destabilize a world to be non-prime, or even if it did, it wouldn't be to such an extent as in the so-called "trivial world."
Maybe that's just the impression that I get. But if not, if it's less drastic for a world to be non-prime than non-consistent, then maybe we can imagine worlds that are just partially non-prime, where some-but-not-all disjunctions satisfy the pertinent condition. If it is possible for there to be an "impossible" world of this kind,◊ even one that can be inhabited by real concrete beings, is this the kind of world that would be needed for hard free will to exist (there)? I ask because of the following from Berto[17] ("Impossible Worlds and the Logic of Imagination"):
However, to achieve [our goal] we do not need nonprime worlds such that [disjunctions are true apart from any of their disjuncts being so]. What does justice to under-determinacy in imagination is the plurality of worlds accessed via RA or fA : different worlds fill up the unspecified details in different ways. [emphasis added]
So we have that non-prime worlds can figure in a representation of "under-determinacy," and though this is not said to be relevant to imagination, yet insofar as imagination and will are not completely identical (though they be much involved, the one with the other), do we have that the "under-determinacy" involved in hard free will is related to the matter of non-prime worlds?
◊I wanted to think of "demi-possible" worlds, but per Conifold, if ◊ is idempotent, then d◊ = ◊, which means that possibility isn't properly demifiable. And on some reflection, I can conveniently acquiesce to the claim that ◊ is idempotent.