Or maybe "Possessive Ghosts," but that seemed more confusing.
The idea I have here is that few ghosts should have the ability to interact with the physical world directly, but most would be able to do so indirectly, by possessing objects and maybe people.
1. Possessing Corpses:
Ghosts could possess corpses and body parts, ideally their own. They would then be able to interact with the fortress as though they were rotting, probably missing-several-pieces dwarves. This could be Fun, if only for the miasma. It opens up a bunch of possibilities, from sufficiently happy ghosts returning to work more and enjoy the fortress (assuming they STAY happy, and woe betide us if they don't...), to the bones and spirits of the dead warriors of the fortress returning to arms to fight some deadly force before returning to their rest, to the ghosts of dead invaders possessing their old bodies or random body parts of random animals or whatever to aid a siege. To say nothing of if a dead baron's body is possessed by an angry peasant to make amends for hurts sustained in life...Or, better yet, if someone possessed the dead body of a titan/dragon/etc worshiped as a god...
2. Possessing Objects:
If a dwarf can't or won't possess an appropriate corpse, all sorts of other options might be available. A dwarf that just wants to be with its loved ones might possess a figurine and walk around to find them; a dead soldier whose body is unavailable might possess a statue to attack the enemies of a fortress (bonus if it was a bronze colossus). Dwarves could theoretically possess all kinds of objects. A dead moody dwarf might possess a rock or a bar of metal or something, moving it and maybe hopping from object to object, until made into something, permanently binding its spirit. (Moody dwarves might prefer possessed crafting materials, which could get interesting if you Planepacked them.) Elves who died at your fort might make a last stand against your use of wood by binding their spirits to logs, which are, again, permanently bound to anything you make out of them. (Please consider the uses of filing your dormitory with beds literally full of the vengeful but impotent spirits of elves trying uselessly to convince you of your Wrong Ways, and don't just burn them. That might release the spirit, anyway.) Dead kobolds might possess gems or crafts or whatever, which then try to roll or slither or whatever off the map. Vengeful goblins might possess the weapons or ammunition of your dwarves, requiring exorcism or powerful-willed dwarves to deal with. Conversely, dead dwarven soldiers might possess the gear of living dwarves to lend them advice in battles, temporarily increasing their skill and helping them learn faster. Once tools like metalsmiths' hammers and masons' chisels and such are implemented, they might be possessed by dead master artisans; until then, workshops might be. Naturally, dwarves might possess their own artifact on death, or a named weapon, leading to the voices of dead dwarves giving advice to the living.
3. Possessing Creatures
This is where it gets fun. A possessed creature, depending on its will and the intentions of the possessor, might be almost unaffected but with the knowledge the possessor had, or completely controlled with his memories open to the possessor, or somewhere in between. Obviously, all kinds of dead soldiers might possess living ones, whether their own or the enemy's, although only champions would have any reason to possess their own recruits. A lonely child or whatever might possess a dog or other animal, which could be cute, and interesting if the ghost could get possessed by a mood...Dead kobolds might possess children, causing them to steal stuff, run away, and get captured by kobolds, who might use them as thieves against you in the future. Dead goblin snatchers might possess children to kidnap babies and run, catching two or more kids in one fell swoop. A dead dragon might possess a dwarf to try and get revenge, then repeat when he tries to path off a cliff and kills his host. A dead dwarf might possess a dragon, and try to get people to worship it. The possibilities are endless, even before considering vermin...which could be eaten by cats, or even starving dwarves...
4. Other:
Creatures could possess areas, causing interactions in those areas. A necromancer that dies might possess the area nearby and reanimate any bodies in that area as long as they stayed there. A dead elf might possess an area of woodland, giving advice to passing adventurers and animating trees to act as guardians when needed. A dead miner might possess areas where noob miners are working, to help them mine better. Or maybe that last one's not such a good idea.
Elves, and maybe other creatures, could possess plants. A massive, motile oak would be terrifying, although if it chose to uproot a kobold bulb instead, that would be...pathetic.
Possessing mechanical devices seems possible, whether it's a bored dead pump operator giving random machinery a spin (possibly triggering your complex water-based trap, possibly powering something useful) or a dead berzerk dwarf possessing random levers/pressure plates/etc and triggering the randomly, or possessing doors/bridges/etc and making them open or close randomly...
I'm not sure how these would be implemented, but possessing fluids and natural features seems cool.
Any other ideas for possession? Any of my ideas you don't like? Etc? It just seems weird to end a suggestion with the end of the suggestion, for some reason...