Shock resistance (possibly only extrinsic shock resistance) prevents you from being paralyzed by a ghost’s sudden appearance even if you lack free action, because you are resistant to being (emotionally) shocked by things.

Cloudy and white potions, in addition to milky potions, sometimes contain a ghost.

If you attempt to move into a ghost’s space (either if it’s invisible and unseen by you, or by m-moving, not by moving normally towards it when you know something’s there which would still be an attack), you successfully move there, displace the ghost to an adjacent space, and take some damage from creepily passing through a ghost. If this is implemented in an extra evil way, this could be cold damage that destroys potions.

Ghosts (and technically other player remnant monsters such as wraiths, ghouls, etc) found in bones levels are empowered with additional abilities that normal monsters don’t have.

Ghosts can possess other monsters. When a possessed monster is killed, the ghost pops out again.

The game tracks players with the highest death counts, and ghosts sometimes pull their names from the top 10 of this list.

Ghosts and djinni can emerge from any bottle, not just potions with the randomized smoky and milky appearance. The rate at which they emerge is cut accordingly.

Ghosts left behind by coaligned players are peaceful and grudge against hostile-to-the-player monsters. Ghosts killed by brainlessness do not behave like this, nor do ghosts not generated with bones.

Artifacts can always hit shades (and ghosts in variants which treat them similarly), even when non-blessed.

Ghosts, shades, and any other incorporeal monsters can’t wear armor or wield weapons. (With a possible exception if it’s cursed.)

Make it so that you cannot rename ghosts. (The ability to rename ghosts makes a bunch of otherwise interesting ideas about interacting with a specific named ghost boring, because you can just rename any ghost to whatever you want).

Incorporeal monsters like ghosts and shades are only hit with blessed objects, artifacts, magical attacks, etc 50% of the time they would have otherwise. The only items that are guaranteed to hit (assuming you make the to-hit roll) are those made of either silver or bone.

Some mechanic for Valkyries that involves escorting the spirits/ghosts of those who die in battle.

One take on this idea: you can chat to ghosts to tame them, and then they will follow you around as pets do. Then you can release their spirits by praying on an aligned altar. Doing this successfully gives you some sort of reward. However, hostile monsters will attack the ghosts you are escorting without provocation, making this not trivially easy.

Ghosts do not count as undead; they’re just dead.

Add Dudley to the list of random ghost names.

New artifact dagger The Barrow-blade of Cardolan: has some flat bonuses, some additional bonuses versus undead, and instakills all W, ghosts, and shades. It is either lawful or unaligned. Could also be a short sword, since there are currently no artifact short swords, but for flavor it should be a dagger.

Spell of necromancy that turns a corpse you’re standing on (or possibly adjacent to you) into a tame undead. Probably in the clerical school, though a case could be made for matter (and certain variants have a school of necromancy which this would much more directly fit into). It will work on corpses of undead that you have killed. Casting this spell would have some sort of penalty for lawfuls; there would be some sort of warning before trying to cast it.

You can normally only create zombies or mummies when an undead monster exists for the kind of corpse you are animating (i.e. a jackal can’t be turned into undead but a gnome can turn into a gnome zombie/mummy). For intelligent monsters that have no counterpart, the spell can create ghosts.

At a high enough skill or caster level, you can create skeletons from the corpse of any vertebrate, and maybe even vampires from humanoids. You can also create wraiths and shades from monsters that have no undead counterparts.

Chatting to the ghost of a dead player may tame it. At least one of your role, race, name, or alignment must match the ghost’s, with an increased chance the more factors match. If the attempt to tame fails, it may become peaceful or remain hostile. In either case, subsequent chatting attempts won’t work.

Ghosts cannot be touched with most items, or the player’s body, similar to shades (probably blessed + silver objects would suffice; though silver does not sear them like it does shades). They don’t need to have an amazing AC anymore because of this.

You can apply a cursed bag of holding (or possibly empty bag of tricks) at a ghost to capture a ghost, which can then be used to recharge a wand of undead turning.

When you get lifesaved, a ghost with your name may be created next to you.

Ghosts lose their physical touch attack in favor of a reusable attack that scare-paralyzes you. Possibly, if this is unflavorful due to it just being the same ghost, change their AI to make them turn invisible after a certain amount of time being visible and not next to the player. Then they path towards you, and if you either stumble into them or they are next to you and get a move, they turn visible, paralyzing you. Since the actual action of them appearing out of nowhere would be what frightens you, see invisible would negate the effect.

The player’s own ghosts generate peaceful (possibly only in bones files).

Better ghost scaling in general. Ghosts should have a wide range of strength levels, and should be capable of posing problems even to late game characters. The more ghosts you summon by repeatedly entering a haunted temple, the stronger they get.

There is some sort of way to store a soul (ghost, or monster’s or pet’s soul from its corpse before it rots) in an item, and later release it into a freshly killed corpse to make the soul inhabit that body. Possibly this could be a ritual spell.

Allow taming of player monsters in bones with one prerequisite: you must tame its ghost, draw the ghost over the corpse, and then revive the corpse. To facilitate this, killing a player monster should always generate a ghost over the corpse, possibly with a gravestone and 80% of inventory being cursed.

Special room “ruined church” that contains ghosts, pieces of glass scattered around the floor, maybe an altar, maybe a spellbook, and blank paper/scrolls of junk mail scattered around the floor.