I would like to see encounters where players are made to change up their tactics, spell loadouts, and roles that are used in a frontal assault type approach to dungeons.
Creatures in certain areas of a dungeon that devour magic, possibly with a dispell on hit type mechanic, and are highly resistant to magic if not immune from it. This could cause players to cycle in the melee members of their party to get as many hits in as they can before all of their buffs are gone and run back to casters in the rear to get new buffs while somebody else steps up to hold the creatures off. It would also have the effect of causing casters to memorize more buffs and fewer offensive spells.
In another encounter it could be the opposite. The creatures are so hardened that blows and piercing from weapons and arrows simply bounce off. It is a rush for the casters to kill the creatures with magic before the melee people who can only take the blows of the creatures fall.
Ranged only encounters. A place inacessable to players while creatures are "alive" there. Perhaps they players would have to range attack down monsters before a door will open, or a bridge will become accessable.
Creatures that require special tactics, like rust monsters, and oozes always shake things up. Perhaps a creature harmed by a certain damage type (physical, magic etc) would explode causing an AoE effect. It could even cause a clone of the same monster to come into being from the remans of the exploded orignial. It would require players to figure out what damage type is good for the encounter and which one is bad. I don't know the guts and gears of how the game works, but maybe the dungeon or different areas of a dungeon could have creatures randomly generated with different damage types that will set them off. Physical, magical, fire, electrical, acid, negative, sonic, etc could all be a hinderance or the key to victory in the dungeon in a random fashion. Something to break up the standard equipment, consumable and spell load outs people use all the time.
A few ideas. I have more, but I'm not sure this is exactly the sort of feedback you're looking for.