Could the Word have incarnated a female human nature?
I say the Word could have done so; whether or not one says that such a thing would be fitting, such an act is within the range of God's absolute power.
Addendum, in case what should be obvious is not:
As Christ could have been Christa, say, there is no obligation on the Church's part that a bishop or priest be male; Christa could have represented humanity as well as Christ did.
One who nevertheless obstinately holds that only males can be ordained to the episcopate--that there is an ontological block to female ordination and not merely a block in fittingness--is committed to denying the absolute power of God, and that implies a commitment to denying God.
The result, as there is and can be no other God, is that any who deny that women can be ordained--as a matter of ontology--are committed to atheism, which to say: heresy.
Still, there is an argument to be made for restricting ordination to males resting on the symbolic value of having humans who cannot bear children mediate grace to the laity. Their very barreness serves both to elevate the grace-filled fecundity of Mary as a model for Christian virtue, and to point away from the male clergyman as a model. However, that argument implies a block not of ontology--as if women could not be ordained--but only of fittingness. For some congregations at some times in some places, restricting ordination to males serves better. But that restriction, enabled by merely contingent, historical circumstances, is itself entirely contingent. Thus, to turn it around, one might imagine circumstances under which ordination should be restricted to females as a contingency. More to the point, it becomes entirely intelligible from a critical point of view how the Church over time should have moved from restricting ordination to males to opening ordination up to females.