The best kind of antagonist is the one that could have been you in another scenario. An antagonist you can justify and somewhat sympathize with, if it wasn't you vs. them. Something like a barbarian nation fleeing their homeland, or a pirate nation who have been forced ashore by a fierce leviathan below the waves, and have taken to conquest to sustain their need for a land and home. Maybe they could be made up of players, but supported by melee/archer-based mobs in war claims and for defending their holdings. Perhaps they speak a different language, but could feasibly listen to reason, or maybe players could infiltrate them... Just so much better than beaten-the-horse-so-much-its-basically-glue cliche of dark forces.