Monster Prefixes
ADOM has a lot of monsters, all essentially identical. What I think would be cool would be if random monsters could occasionally have random prefixes/suffixes that give them extra abilities that might make them more dangerous/interesting. So, occasionally instead of running into a Goblin Rockthrower, you might run into a Noble Goblin Rockthrower or a Deadly Goblin Rockthrower.
Here's some ideas:
Humanoid Prefixes
Noble: Forces alignment to lawful, alignment penalty if attacked
Murderous: Greatly increased critical hit rate
Armored: +20 PV
Spritely: +40 DV, +20 Speed, -50% HP
Lumbering: -50% speed, -10 DV, +10 PV
Corrupted: Corrupting attacks
Deadly: Attacks do double damage, one extra attack per turn
Regenerating: Regenerates health
Karmic: Karmic property
Wealthy: Drops extra loot
Sorcerous: Gains the ability to cast a random spell (except death ray)
Daemonic Prefixes:
Diabolical: Double HP, attacks inflict paralysis
Elemental: Immune to all elemental attacks
Enervating: Attacks drains a random stat
Maniacal: Summons random monsters
Petrifying: Petrify breath attack
Undead Prefixes:
Ethereal: Passes through walls, +20 DV
Spectral: Permanently invisible
Possessed: DV, PV set to 0, 3 extra attacks, never flees
Withered: -20 Speed, -50% HP, attacks inflict Slow
Construct Prefixes:
Inert: Immune to all elemental attacks
Gargantuan: 10x HP, attacks do double damage, -50% speed, -50% to hit
Reinforced: +30 extra PV
Enchanted: Paralyzing skin
Hexing: Drains PP
Gem-encrusted: Drops a huge pile of gems upon death
Beast Prefixes:
Ferocious: DV reduced by half, all attacks inflict bleeding
Diseased: -20 Speed, -10 DV, attacks inflict disease
Venomous: Attacks inflict poison (and already-poisonous enemies are stronger)
Primal: +10 Speed, +10 DV, higher critical hit rate
Dragons Prefixes:
Eternium-scaled: PV is tripled
Fearsome: Never flees in battle, penetrating attacks
Wizened: Gains a random spell
Toothless: Attacks do 50% less damage
Distorted: Doppleganger confusion