Future expansions.

it seems that "Shades of Darkness" turned out to be a successful expansion. which faction will be the SEVENTH?

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