The beauty is that they gave us the same editor they used to make the game so, theoretically, we can add whatever we want to any part of the game, including GM mode. You might be interested in checking out my mod. It adds *almost* everything to GM mode character wise and I'll be adding the remainder of NPCs eventually.

GMAddon1: Adds almost every single monster and NPC from the main campaign into GM mode, plus a few large custom maps to use in GM mode.