Random thoughts from the peanut gallery.
Obviously, since it is me, I would prefer psionic flavored versions of all the base classes to be the default for Athas, lol. But that’s not useful for this thread so I’ll skip over that for now.
Paladin.
The original theme of this class was faith and devotion to a god who embodied the concept of the medieval version of lawful good. This was later expanded in 3e to include the non personalized concept of lawful good, and expanded again in 3.5e to all four major extremities of the alignment system allowing for evil, chaos, law, and good type paladins, with or without worshiping a being.
In Athas, most of the SKs fit this archetype, to be honest, via their lust for and devotion to the concept of power.
Druids could also fit the concept, and in fact might be the best tie in for paladins, right along side with genius-loci, as a source for becoming a paladin.
Imagine if you will, an elite order of consecrated eco-facist warriors, empowered with the strength of Athas to fight against defiling everywhere, and to use the corpses of defilers to re-enrich the land and literally return what they stole to the soil. Give them the ability to dedicate and consecrate the grave of a defiler to become a nexus of growth and nourishment to the land.
Sounds very Athas.
Monk.
Much like certain oppressed peoples here on Earth turned their native martial traditions into dances and adapted farming tools into martial forms when threatened by militant governments, I think it completely appropriate for there to be such a movement among the oppressed lower classes on Athas.
Furthermore, since Bards are such movers and shakers, plus being poison makers, (hah, rhyme intended) having a secret martial order/religion of hassais/hashīshīn/assassin monks who are used as convenient cleaners of the political landscape, with or without intoxicating poisons should have already existed. Bards are smart, using poisonous disposable tools that can’t be traced back is obviously better than poisoning someone themselves.
Thus I could easily see at least two martial traditions easily existing on and fitting in with Athas.
Not to mention that elves could easily include something akin to a shaolin long form into their running techniques incorporating whirlwind strike and cleave effects into run by attacks?
Plus since everyone is supposed to be psionic by default, having some sort of psionic shadow walk trance run should totally be a thing for elves.
Barbarians.
That was seriously an argument? I was too busy playing regular D&D and futuristic Scifi RPGs and Supers RPGs, and reading scads of books to notice all that, lol.
In any case, barbarian, brute, gladiator, whatever… call it what you will, it still hacks and bashes just the same, and Athas needs it. If you don’t like the rage mechanic for story reasons (were there really no raging characters in the novels?) then use the Sohei Ki Frenzy mechanic instead.
Or allow both, and give them both as a class option, the same way rangers pick between two weapon and range feat chains.
Soulknife
I think the Athasian Soulknife is a better design by far than the base class version. I am very fond of it and use it in several of my character builds, sometimes with the Soulbow.
While the class and concept is very cool, the Soulknife really feels like a highly educated elite sort of prestige, maybe something limited to certain nobility, or perhaps exclusive to the Tarandan school of psionics. (Did I spell that right?) From what little I recall of lore, that school was supposed to be somehow superior to all the other psi traditions in Athas? Knowing the trick to solidify energy sounds like one such possible superiority.
If there is one flaw, I think there should also be a Soulfang branch of the prestige usable for psionic animals and psionic plants to enhance their fangs and claws and vines and thorns with a few simple enhancements… which would also work nicely for psionic monks and unarmed strikes.
Perhaps an alternate ten level prestige that focuses more on simple abilities… such as take away the thrown blade and psychic strike damage and other more complex abilities, and replace it with a simple defensive boost (toughen skin or basic armor via solidified energy), natural weapon effective size increase, and some boosts to skills like jumping and climbing and boosts to movement speed?
While I doubt anything like this appeared in the novels, it sounds to me like a very logical evolutionary adaption in a world like Athas.