New material “demonite”, a metal which causes silver-like searing to A-class monsters, white unicorns, and some other “good” monsters.

Unicorns are more likely to generate peaceful if you are celibate and have not yet dallied with a foocubus in your game. (Since coaligned unicorns are peaceful anyway, this would affect cross-aligned ones only.)

If you polymorph uncontrolled while wielding a unicorn horn, you may become a unicorn corresponding to your alignment.

When you throw a gem at a unicorn, it may vanish entirely, like a nurse does, instead of teleporting somewhere else on the level. This makes it unreliable to farm luck from them indefinitely and still have them around to kill later to get the horn and all the gems back.

The chance of vanishing could even be based on the number of gems it is already carrying, so that it won’t or is unlikely to vanish after throwing it a couple of gems but gets more likely after giving it lots of gems.

When the player does something that decreases the tameness of a pet, the amount of decrease should be doubled if the pet is intelligent and “weakly” cross-aligned (either the pet or the player is neutral and the other isn’t), and tripled if the pet is intelligent and “strongly” cross-aligned (the pet or player is lawful; the other is chaotic).

Unicorns always count as “intelligent” for these purposes (they’re not intelligent, but fundamentally understand alignment).

Track the amount of Luck increase you have had from a unicorn. If you steal gems from such a unicorn or kill a unicorn you have gotten Luck from, immediately revert that Luck increase.

Assuming unicorn-gem interactions are dropped or otherwise made alignment-agnostic, remove gray and black unicorns entirely, and leave white.

If a peaceful unicorn sees you wielding or applying a unicorn horn, it gets angry.

Unicorns (possibly only coaligned) can be tamed by throwing them a specific gemstone, which is deterministic for each individual unicorn and is random. Or else keep it simple and make it a low-ish chance per gem thrown.

Unicorns can only be tamed/ridden if you have never consorted with a foocubus.

Unicorns of different colors grudge each other.

Penalty (alignment? luck? aggravate monster?) for eating a coaligned unicorn.

If you’re polymorphed into a unicorn, you can dip your horn into potions.

Unicorn corpses are never tainted from age, though they do eventually rot away.

Unicorn corpses have a chance of conveying teleportitis.