According to Tibian lore, the Isle of Mist can only be accessed by druids as this island was a meeting point for the corrupted druids of Carlin (Carlin is a city of Druids). They worked along with the necromancers and monks on Zathroth's behalf.
This is the reason why it can only be accessed by druids, since having that vocation filter makes you understand the history of the place and who this meeting place belonged to.
Unfortunately there is no other place with a single boss for a specific vocation. It's very likely that Cipsoft has decided not to continue this kind of mechanics, since it could be seen as unfair for other vocations.
We have to keep in mind that at the time this respawn was created, Tibia was still a relatively young game and decisions were made by far fewer people than now.