I feel as if The Mini-Boss NPC is such a rare commodity. I rarely ever run into one, and when I do it’s very underwhelming at that. It’s simply a somewhat harder version of the dark wizard NPC. I think it would be a cool addition if they gave very rewarding though very difficult quests to a person of the same reputation. These quests would be a good source of crowns, experience, reputation, and most of all; fun.
Their dungeons or spawn are also quite simple. Maybe have more variety with them. Instead of a simple roadside, or forest dwelling keep. Mountain homes, and ships would be a nice place to see them. Providing a little extra challenge if they happen to be surrounded by guards, and fiends alike.