Dark Elixir Troops
This one is a given. For over a year the Dark Barracks has said "coming soon" on the squares where buildable troops usually are. That's because the developers haven't announced them yet, and we don't have a troop to fill that square. New troops to change how we attack is in the works in one form or another, and this is likely one of many changes. That said, we've seen the same coming soon squares for a while, so don't get too excited just yet.
At some point in the near future another army troop or two will be added. And we have no clue what it will be. The regular Barracks has the goblin that favors loot, and each troop targets specific things, and we're assuming that now they've released the Lava Hound that targets air defense units, a Dark Barrack troop that targets loot will be next. It could be something completely different, but just know at least one more Dark Troop is coming in the update this November.
There are also multiple rumors floating around that Supercell will be adding a fourth spell to the Dark Spell Factory. Hopefully all of these questions are answered soon enough.