'From the depths, a new threat rises.'
I would love a merfolk army. Something about them being tired of having their water ways corrupted, and moving to take revenge on the surface dwellers.
They could be an Order army, but I would love them to be in Destruction. They are not necessarily 'evil', but they are tired of being pushed around, and have taken things into their own hands.
Kraken, Sea serpents/dragons, merfold infantry, creature of the deep abyss... so many possibilities.
Bonus points if there are merlock infantry. wahblurrglegurgleworgle!