Central to Plantinga's free will defense is his notion of significant freedom, which is, roughly, libertarian freedom with respect to moral actions. Plantinga attempts to capture a necessary condition of significant freedom in terms of the actions of a given agent at different possible worlds. Very roughly, an agent S is significantly free with respect to a moral action A only if there is a metaphysically possible world W at which S performs A and there is a distinct metaphysically possible world W' at which S does not perform A.
I worry that possible-worlds analyses of libertarian freedom -- like possible-worlds analyses of most things -- are too coarse-grained to capture the essence of the notion. So, for example, I'm inclined to think that an omnipotent and morally perfect being could be significantly free even if there is no metaphysically possible world at which such a being performs a wrong action. It seems to me that such a being would have the power or the ability to do wrong; it's just that such a being would never exercise that power, in virtue of their moral nature. To use a term common in the literature on dispositional properties, the ability to do wrong is a "masked" disposition in God.
Similarly, it seems to me that a morally virtuous created being could be significantly free even they never perform a wrong action in some metaphysically possible world. Like God, such a being would have the power or ability to do wrong; it's just that their moral nature is such that they would never exercise that power. As with God, then, the ability to do wrong in such creatures is a "masked" disposition.
But suppose someone is unsatisfied about all this without the prospect of a "worlds" analysis of freedom. Fine. Hyperintensional analyses are all the rage these days, and so one could appeal to impossible worlds and non-trivial counterpossible conditionals to account for the sense in which such beings have such an ability.
Is there a punchline? It will take some time to flesh out the point properly, but intuitively, the worry is that if modal analyses of signifcant or libertarian freedom are inadequate, then this will cause problems for Plantinga's free will defense, and for his account of transworld depravity.