This reminds me of an exert from the Road to Urik;
Neeva and Rikus are setting up a tent in the desert; after they got it set up, they both went to sleep. Some hours late, Neeva wakes up and says, "Look up towards the heavens; what do you see?
Rikus replies: “I see millions of stars”.
Then Neeva says, “What does that tell you?”.
Rikus says, "There are millions of galaxies and billions of planets: and the fact Ral is in Hoard, means it is early morning, and it should be a clear-day tomorrow: “What does that tell you Neeva?”.
“That you are dumber than a Mekillot: some one stole our tent”.
The main problem with Illithid being on Athas is another threat that I don’t think was ever handled well.
The Psurlons.
They supposedly play the githyanki and illithid against each other, are supposedly rivals with the illithids when it comes to psionic might, and have had a long vested interest in Athas due to the planet’s ability to take insane amounts of psychic damage. They are supposed to be great manipulators and deceivers (despite what their stat blocks actually reflect).
If you decided that the psurlon fill in a similar niche as illithid on other worlds they would need some tweaking but could very easily fill the role.
Suggestions:
Redundant Organs: Like several types of worms, psurlon have multiple vital organs including brains and hearts. For obvious reasons this would make them immune to critical hits and precision damage.
Chorus of Minds: Due to the fact that psurlons have multiple brains, enchantment/charm/mental control used against them automatically fail. Also anytime that they would need to make a Willpower saving throw they roll twice and take the better result.
Focused Mind: A psurlon may use one of its psionic abilities as a free action once per round.
Breath through Skin: Also like worms, psurlon breath through their skin. This allows them to breath inside earth or silt without difficulty. This also makes them extremely vulnerable to gaseous attacks and poisons (-4 penalty to save) as such attacks enter directly into their circulatory system.
The psurlon were originally something else entirely. Something likely more human. They have the ability to start changing others into proto-psurlons with psionics and grafts. Grafts which are likely the psurlon’s own removed organs that will regrow once they fully heal.
Throw in the fact that the whole race wants to merge their mental power into a psionic gestalt…