After a boss has been beaten for a quest, it's nice to be able to revisit them, battle them again, whatever.
However it's a pain to have to walk/ride/fly out every time.
Since the boss icons still remain on the map in the spell book after the initial battle is over, how about making those icons become teleport links. The way this would work would be:
you open the spell book, open the map, find the boss you want on it and click on their icon. The game would move you to the sigils outside of their lair.
Obviously you'd have to be IN the area with them, not in some other area, to start with.