This suggestion is for Ghosts in fortress and adventure mode and adding more fluff in general to how "evil" lands work, and many other general stuff for undead
My idea is the addition of a "spirit realm" which is generated alongside the "mortal realm" the spirit realm and mortal realm are seperated by one "S" layer, in order to move along the S layer you have to die, usualy at least, your body is left behind in the mortal world, dead. In the spirit world you are reincarnated as a "spirit" which has the same personality traits as the dead creature. a spirit cannot usualy interact with the mortal world but there are exceptions.
-The features of the spirit realm-
The spirit realm has its own unique biomes, there are areas which are "heavenly" and areas which are "hellish" along with many other areas such as "dreamlike" and "Feylike" there are even kingdoms within the spirit realm ruled by the gods worshipped by mortal races, if you worshipped a god as a mortal an emmisary of the god will lead you to the kingdom. If you were not buried in the mortal world however you gain a stigma from all spirit creatures, a merciful god will however let you return to the mortal realm in order to haunt people until you are buried.
-Dieing in the spirit realm-
when you die in the spirit realm your soul is returned to the spirit kingdom which it belonged to, however with each death in the spirit realm you lose more and more skill points, you also cannot gain skillpoints in the spirit realm, eventualy, a player is so weak that he basically has to "rest in peace" and start a new adventure
-Evil biomes-
in the spirit realm, areas connected to evil biomes allow easy transport from the spirit realm to the mortal realm by possessing bodies, the evil areas of the spirit realm have animalistic spirits that enter and reanimate a corpse, however any spirit can enter a body and use it for good or evil if they are in an evil biome, such creatures however can never leave the evil biome lest their soul is purged.