Having realized that it was no esoteric question, I put Karl Rahner and Thomas Aquinas into direct confrontation with one another, because I realized I couldn't let Aquinas's position enjoy the renaissance it is currently experience. Thomas reasoned that, given the common power of the Persons of God, that any one of them could have become incarnate in history, and that it didn't have to be the Second Person, although that it was "fitting" that the Word should become human in Jesus of Nazareth. Recalling that this odd category of "fittingness" was something that went back to Anselm of Canterbury's book on the Incarnation, I followed up on what exactly that might mean. Then I turned to the 20th century's great genius, Karl Rahner, who had broken strongly with Thomas on this point, insisting that only the Word could have become incarnate out of the Three Persons. It sounds esoteric, I suppose, to people not prepared for Trinitarian theology, in the same way that quantum mechanics isn't something that one can just pick up from talk on the streets. But what I realized was that Thomas's position, followed in its implications, actually ended up effectively denying any possible knowledge of the Triune God, which is certainly incompatible with what Christianity has held. Ultimately, for Christianity to "work," only Rahner's reasoning worked: that the distinct characteristics of the three Persons of God are part of their essence, and essential to our experience and knowledge of them, and that they are not all effectively "interchangeable" because of their unity as God.
I think it's actually the most important piece of theology I have ever written, other than the dissertation, and so it's off to months of silent appraisal once I get it over to the post office.