There are some old Fantasy-Novels with "Demi-Plane"-Dungeons:
In the Death-Gate Circle the "good Overlords" imprisoned a powerhungry Wizard-Clan in a Dungeon-Demiplane wich should educate the Wizards; over a few hundred years the overlords vanished and the Dungeon turned into a Deathtrap wich tried to kill the Inmates. This forged the Descendants of the Wizards into hard-hearted powerful Warrior-Mages. The Story of the Books begins when the first Prisoner escapes and tries to free the rest of his clan.
In the "Dungeon"-Series there is a Dungeon that connects different Worlds and IIRC also different times; sometimes People just vanish and appear there. It is a harsh environment and creatures from different Worlds and times had to work together to survive.
Underwater Areas are used a lot by World of Warcraft. There is even a sunken temple; it could be interesting, but Potions and Spells of Water-Breathing make the Water meaningless. In the old AD&D Games (i.e. Dark Queen of Krynn, Dungeon Hack) some Spells worked differently (or not at all) underwater.
Difficult to top WoW in Areas, from the South-Sea to giant Insect-Hives to flying Islands they already used everything
(Edit: I know that nothing of this is really new, but I guess most younger Gamers will have seen it there first)
Some Mountaineering might be interesting; the use of Ropes and appropriate Clothing on the Way to some long forgotten Mountain-Monastery might be a challenge - or boring and frustrating (IIRC in Ishar 2 there was something like that and it wasn't entertaining). Flying Monsters might be a big Problem in such areas.