1d30 sets out in a recent post to define classes by what they fight. This differs from the definition of classes by how they solve problems. If you look at classes by how they solve problems, you get Fighter (solve problems by physical force), Mage (solve problems by magic) and Thief/Rogue (solve problems by skill or finesse). Under this paradigm, a Cleric/Priest class requires a bit of convoluted rationale. Do they solve problems by faith? Divine favor? ???
The Fighter/Mage/Rogue paradigm shows up in a lot of games. Two that I can think of off the top of my head are Warrior, Rouge, Mage (Stargazer Games) and Numenera (by Monte Cook).
Using 1d30's paradigm, though, makes you wonder why a Rogue/Thief is necessary. Fighters are a human class designed to fight just about anything, especially other humans. Mages learn magic to fight demons. Clerics rely on their faith to fight undead. This way of looking at class feels more old-school to me, especially in light of the post shared in his post.
Reading the article reminded me of a couple of the design goals of the ACKS Psionicist. Specifically, the ACKS Psionicist would be different than a magic-user and the psionicist would be useful against incorporeal foes. The latter required me to redefine ghosts as something besides undead.
One of the proficiencies of the ACKS Psionicist allows the psionicist to actually grapple ghosts or shadows. It doesn't allow the psioncist to kill a ghost or shadow outright, but he or she can deter such creatures long enough to protect others. Ghosts/shadows can walk through any obstacle and I wanted a way to combat them that wasn't a variation of creating a wall of force.
Using mental powers makes sense to be a quick method of attack/defense against these creatures. More than that, the most vulnerable part of a ghost is the mind. Since it cannot be attacked easily by physical force (fighter) and magic may take too long to cast (mage), the ability to attack the mind of a ghost fits in very well.
Even astral constructs fit because it is the psionicists attempt to fight fire with fire. Astral constructs and ghosts are made from the same substance, so they naturally are able to fight or contain each other.
From 1d30's post, though, reminded me also of the ghost generator I am working on. Instead of a 6-7 HD creature with aging attacks, ghosts can do a whole range of things from cause disease to cause nausea to causing instant death. Ghosts can also have variable weaknesses ranging from silver (or electrum) to nothing. More powerful ghosts are even immune to +1 magic weapons.
Still, I want to go back and add in creatures that range in HD from 1 to 10 as well as add in various abilities. I don't mean a ghost dog for a 1HD ghost, but instead something like a mindless ectoplasmic blob that was formed from a soul that didn't successfully become a ghost.
More to think about.