Technically, the Inquisitor could have caused the Breach (and have forgotten about it) and not be the main villain of the game, who could be someone trying to take advantage of the Breach for their own ends. It seems unlikely.
That said, I doubt this will happen. Particularly as it seems rather unlikely for any non-mage Quizzies, which is most of them (8/11). Also it would be like Revan 2.0, and as epic as that twist was you can only do it so many times before it gets old, especially since it's basically the best known plot twist in any Bioware game ever ('you are a Bhaalspawn' barely counts, if only because the player can basically miss the twist).
I'd just be happy to be able to get to know the antagonist on a personal level before we confront them, and I hope they are relatable. Loghain only really got humanised if you spared him, and the least said about the Archdemon the better (what can you even say about it anyway?). Meredith was a thematic presence throughout, but her actual screen time was pathetic; the Arishok was a much better antagonist due to the relationship he and Hawke built up throughout Act 2. The fact you don't even get Meredith's backstory unless you side with her in Act 3 is particularly egregious. Thus, whoever the villain is, I hope we get to know him/her quite well.
It's probably Cullen.