... Maybe there could be a few conditions on whether the ghost is summoned after the character's death?
You could perhaps consider any creature with significant intelligence as having a soul too?
A good criterion. I think that mistresses would make great ghosts too.
Re chao, there could be a chance for ghost to spawn from a specific death of a character. Also, levels could have a flag like "AllowGhostsToSpawnFromDeadChars = true" type of thing. There could be a cooldown timer between the time of creature death and the time when the ghost shows up.
Should the ghost show up on the level where they died? Or can they show up at any level where you happen to be? Should they show up where their corpse is?
You could offer a dead creature's corpse to a god to prevent the ghost from rising. (not every creature would have a soul, but say if you wanted to be sure that Sherarax didn't show up in ghost form...)
Ghosts could arise out of their former avatars being killed in a certain way (beheading?) or there could be simply a 10% chance of them showing up at all, after the death.
Engraving could be modified to require a tool, like a pen or something. Then you would dip the pen in some ink, and it would "engrave" onto the surface of the floor. Other ways to write would be by engraving with a tool that is harder than the floor on which is being engraved.
I introduce that idea because I thought it would be cool if ghosts could write threatening things in blood all over the dungeon floors.