Future expansions.

Most likely, the Mages. I believe there are several reasons for this. The Elves are a dormant race. Until one of their kings, they essentially had no foreign policy at all. And considering that the Mages will soon have many clashes with the Necromancers, they will be a prominent race. Unless, of course, these events are moved to Heroes 7, which is more likely for me personally. The Elves simply have nothing to do in the game's plot.
And I also saw drawings of creatures from the Academy somewhere.