Well, we've been told that the big fight will be someone that we don't necessarily expect it to be, so I think that rules out both Zenos and Elidibus. The Crystal Exarch, Venat and Theyler are my best guesses at the moment.

As for the presence of leafmen in Lakeland, that could potentially be explained by the pixies showing up to help in the battle and turning people into leafmen. They've done as much before after all.