I know misty mountains is a long way away... but here is my suggestion. In the hobbit the orc army that came from the mountains for the Battle of the Five Armies was, like Sauron's army that sieged Minas Tirith, afraid of the sunlight. In Mordor's case for the siege, Sauron was able to put a darkness upon middle-earth. In the Hobbit, Tolkien speaks of a cloud of bats that were probably used to shield the Orcs from the sunlight. While in the movie this is misinterpreted (in my opinion), as bats that are actually used in the battle, what I think Tolkien meant was that there was a literal CLOUD of bats, to make the battlefield dark so the goblins could fight on during the daytime. So my proposition is that instead of the old darkness power Goblins used to get, Misty Mountains instead gets the power Cloud of Bats, which does the same type of things as a darkness for Mordor or fell rain for Isengard. There are some limitations to the idea, such as the game engine not being able to handle bats over the entire map. But maybe it is possible to give an illusion of bats covering the entire map a without doing something that would slow the game down. What do you guys think?