It's possible that the `requires lightbringer ritual' is the best way to do ultimate abilities for each Dark Name. Like you said, it's got a cool strategy element to it, where you force the enemy to give you the tools you need to destroy them. Maybe I'll change Winter's Scythe, then add a few other abilities to the other Names along the same lines.
Regarding new ways to ruin the world, my favourite part is definitely watching the map change in huge ways. The obvious next way to introduce would be to have the opposite of Winter's Scythe, a Name which turns the world into a giant desert. I've not come up with any good abilities for a Name or for an Agent based around that, yet, though.
The plague doctor probably needs changing, a bit, so they also have a plague. Weaker than the Red Masque, but it would synergise with their undead armies. Might need to nerf the undead a bit, just to keep them from being the ultimate weapon all by themselves.