In the tradition of rogue-likes, I'd love to see the Apparition monster class.
Usually you run into the ghosts of deceased adventurers, but this time it's their turn to rob you of your hard-earned loot! Avenge your former rogues of glory and fame by entering the mines as an apparition of spelunkers-past!
There would be great potential for fun skills and abilities (first ones derived directly from in-game ghosts):
Ethereal blast – Wounds and weakens.
Tele-swap – You know it, you want to annoy monsters with it.
Manifest – Take on a corporeal form! When ectoplasm just isn't enough, you can call up a memory of your long-since dead body to punish monsters and friends alike. Just remember, even a seasoned ghost finds manifestation taxing and can sustain it only for a while.
Wail – People mocked your singing in the shower, but now your highly trained off-pitch keening can sap the strength of nearby monsters. An accomplished wailer can make his/her scream cause actual pain.
Induce Slumber – Make your victims fall into an enchanted sleep! Just be sure to get them before a prince ride-by-kisses them awake. Also, your focus might not extend to holding more than one target asleep at a time.
Haunt dreams – Use a sleeping victim's vulnerability to feed your hunger for terror, life-force and just feeling good about yourself. Incidentally, ghosts can sustain themselves on this fear alone. Caution: more sensitive monsters can die of fright when haunted.
Possession – Hostile takeover is no more just for corporations! Posses your enemies to wreak havoc! A forced evacuation from a body can be harmful for your health, though.
And possibly many more… 