The only thing I'd care about having respawn in dungeons are the seeker larvae, but you can farm them at the Mistlands boss too.

The Ghost enemy doesn't drop a trophy or anything else, and all the other "rare drops" from enemies found in dungeons can be found in the overworld as well. Cultists and ulv both seem to have 10% drop chance for their trophy, and rancid remains makes a frequent appearance in skeleton surprise events. Draugr elite trophy can be farmed outside.