My favourite candidate is Galgan, though I think this has pretty much been disproved already. If Demandred was Galgan, it would be monumental, and would confirm a strong alliance with Semirhage in Seanchan. Galgan is also a great general and of the High Blood, so that's very much Demandred's style.

As for dueling, I think those days are over. Rand has decided to balefire all the Forsaken, and he has some sound academic evidence to do so. At best, he and Demandred will role around shooting balefire at one another. But a complex duel with skill is out of the question, because Rand won't take part in such a thing. Even before he knew of their resurrection he was using balefire against Sammael and Rahvin. Demandred, on the other hand, is hesitant to use it.

As I said previously, I'm hoping Demandred and Mat face off as generals with their military knowledge. In the end Mat will kill Demandred - saved by his ter'angreal.