Yeah. What I'd like to see is unique bosses EXCLUSIVELY in quests and give you a one-time experience in a playthrough. Like you know a quest where a village is terrorized by a big famous troll mini-boss in a cave near the village and you are supposed to get rid of that troll once and for all and it won't spawn. The mini-boss should be given a name too like "Drig, The Child Eater" or something.
The other types of mini-bosses that I would like to see is those bosses that are just a bigger version of other same creatures but they don't have a name and they spawn like other creatures but only in said dungeon or place. Like Alpha Wolf instead of Wolf. If you know what I mean.
So, basically Legendary Creatures from New Vegas? They were just ramped up stats, but they were hard to kill. I guess you're saying that you just want to be able to kill a big thing and then celebrate killing it, because it was big and bad. I get you, I think.
http://fallout.wikia.com/wiki/Legendary_deathclaw#Legendary_deathclaw