So this literally came to me in a dream. I don't believe dreams are psychic or anything, it just seems really cool.
In the dream, I let Fate decide my ADOM character... and came up with a Quickling Lich.

The description my "character" had mentioned a gem set in the center of his chest, that was imbued with power by <diety>, and then said he died at <age>.

So here's what I'm thinking:

Quicklings: PROS: High Speed. (200 by default?) CONS: Nearly everything else is average, below average, or low. In particular TO and LE, to make them challenging for any class. Possibly a poor skillset as well.

Liches: Would play nearly completely differently from any other class. Choosing this might well feel like a complete race/class combo by itself. Food and age would both be completely removed as factors, with the character perpetually the age they were at death. The character would not starve but also not eat at all, meaning he or she would get NO benefits from food, ever. Given the power of herbs this seems like it'd be more of a downside than upside. They can still drink potions, but the satiation changes are ignored.
My dream used the lore of liches that requires a phylactery, which means the player would be immortal unless that were destroyed. So that gem would have to roll every time the player is hit by an attack that could destroy a gem. And if the gem is destroyed the player instantly dies. Every melee hit should have a chance to kill the player outright, proportional to their health. At moderate/high levels the chance should be negligible (but nonzero). As the player reaches lower levels of health, the chance raises because the lich can't effectively protect it. After 0 HP, the lich does not die but instead starts to suffer attribute loss, primarily STR and DEX (also AP but that doesn't really matter in most cases) because the damage to the lich's body is becoming so great that parts simply stop working. However, any damaged tagged as "holy" (coming from a blessed source) that reduces the lich to 0 or less hp is fatal, and the lich is "banished by holy powers." And every hit would also be much more likely to destroy the phylactery. If the lich was wearing torso armor, I'd say that only "punches through your armor" hits have a chance to break the phylactery, because otherwise it's protected by armor.
Also, being undead would have one dire downside for the character: blessed armor or weaponry would deal damage to the player every turn it was wielded, and blessed consumables (scrolls and potions) deal damage when used.
I see them as being a bastardized combination of (Chaotic) priest, wizard, and necromancer. The description mentioned that a god imbued the phylactery with the power, not the character himself. Presumably this would be in exchange for serving that god for the duration of your unlife, hence the priest part. The wizard and necromancer should be pretty obvious.
The biggest upside should be massively inflated attributes, especially LE and MA. But all attributes should be really high.
Suggested class powers:
Level 6: Can unequip cursed items at will
Level 12: Aging strike. Any successful melee attack performed by a lich ages the target.
Level 18: Corpse storage. The lich can cast a spell that stores a corpse inside of a gem. This gem can later be 'U'sed to release an undead version of that corpse that obeys the lich. (Gem is destroyed. The only gems that can be used for this are precious, nonmagical stones. Topaz, opal, etc.)
Level 25: Can used uncursed potions/scrolls with blessed effectiveness
Level 32: Command Undead (forces target undead to become your slave. Costs PP and requires successful rolls. Upper-level undead: Very hard to control)
Level 40: +4 to all attributes
Level 50: Can remove specific corruptions at a permanent cost of -10 to an attribute
For XP gain, I'm toying with either very low (lich who's been around, not much more to learn), or very high (recently turned lich, now exploring the fantastic powers available to him/her)

I have to admit, playing a lich intrigues me far more than the quickling. Balance would be an issue, and most of this is whatever occurs to me now. Suggestions are welcome, and if The Creator decides to implement any of this, he's of course got final say.