You invest time to getting different forms of ghosts? Last I remember you just don't heal the creature/hero going into the torture chamber, done. Different forms is a nice novelty and from a developer point of view I wouldn't expect much time, it'd just be about applying the same texture to each already available model.
As Bluto said which helped my point. "A ghost is a ghost, no matter what form." I think that should be kept in everyone's mind when thinking about this.
Now. Why did you mention beetles? Are you saying that you only ever let beetles become ghosts when you play Dungeon Keeper? If so, that's your own personal quirk, thus nothing to do with the suggestion.
Edit:
Just to make a point of interest, applying more possible tactics into the game could be quite useful and ideas for the ghost allows me to make an example. What I mean is, lets say ghosts had a 99% resistance to Physical attacks (maybe full resistance if luck isn't going to be a factor) and their main form of attack was range instead of melee (using the spell Drain-Life) but ghosts would have a heavy weakness to any kind of magical attack (fireball, bolt etc.) then would that not be balanced? Drain wouldn't take much damage (5 hp at best?) but it would still make the ghost useful from a tactical point, like using them as canon fodder to buy some time from all melee creatures. Its a good tactic if the opposing Keeper isn't thinking carefully or watching the battle.
It makes the ghost more useful, but not to a point of being over-powered and the idea of being immune to physical attacks is logical, as long as they themselves cannot fight via melee.