A few things I do for Return of the Witch Lord:
Cold Halls: the green mist will have a more devastating effect, maybe remove all artifacts. They don't need to be destroyed but taken from the heroes only to be found later in other quests.
Cold Halls: Replace the hidden door that leads to the center chamber/spiral stairwell with a locked door that can't be open until the Spirit Riders are defeated. You can replace their secret doors with regular ones. I liken the Spirit Riders to the 4 Horsemen of the Apocalypse, the instructions state that the coffins can be searched and inside each is an interesting find. The 4 Horsemen are War, Famine, Pestilence and Death, each represented by 1 of the Spirit Riders. Inside the coffin of War the heroes find a Battle Axe, searching Famine's coffin will cause 2 of his potions to spoil. The coffin of Pestilence fills the room with poisonous snakes or spiders, hero loses a body point ending his/her turn in that room (Venom Antidote will work to heal). Death's coffin offers a mysterious potion when drank reduces the hero to 1BP but doesn't kill him, whichever quest he drinks it the undead will not attack him.
Gate of Bellthor: add 2 Skull tiles into the corners of the room, every on all of Zargon's turn he may spawn undead into the room to help fight the heroes (Bellthor could potentially fall so easily without help).
Halls of the Dead: turn this into an escape mission, Witch Lord doesn't disappear unless the heroes damage him with a Magical Throwing Dagger, Fire Spell or Holy Water. The Spirit Blade should be stolen from the heroes at the start of this quest. The heroes just need to free their friends and escape to the Stairwell with the Witch Lord pursuing them.
Forgotten Legion: all monsters part of the Forgotten Legion block all s if they roll at least 1 .
Forbidden Cavers: undead only spawn on the corner tiles of the heroes have line of sight on them.
Last Gate: Rediscover the Spirit Blade on the weapons rack.
Court of the Witch Lord: if Skulmar and/or Kassandra survive Forgotten Legion/Last Gate they may stand with the Witch Lord in the final battle.