Mind Flayers - if they would be as travelers, I think, that in this “strange world”, by long-perspective, can be very prone to depression. First, they lost own essence of identity - aren’t unique here in terms of psionics, genius, predation, etc. Under these conditions, they lose the foundation of their ideology of “why the Illithids should rule the worlds”. Next source of depression can be prolonged lack connection with Elder Brains, Ilsensine (or Thoon or other gods) or whole mental network of MF etc. General big alienation problem for more sensitive senses of Illithids.
For me - MF as migrants shouldn’t work in this world, as long-period function with interesant influence on world or economy. BUT beings similar to MF’s as effect of evolution or experiments with ancient lifeshaping, would be interesant. Ofc this should not be simple copy-paste with other origin.
Robots… even as relic from rhulistic civilization, without maintenance, should survive to current times. Collapse and degeneration of ancient halflings should be next step, why robots shouldn’t exist in times of SK’s tyranny.
Migrated mages from other settings - if they come here, they lost own magic. No exceptions - he has to re-learn new magic or go to collaborate with the elements, spirits of the lands or SK. Similar with any function with non-natural long-lived - this is broken and very fast this person die. Even migrated undead can rapid lost own “immortality” - in “normal worlds” false-live of undead is connected with plane of negative energy. Here lack connection with negative energy, so main source “why this still live” is inaccessible. For every spellcaster Athas should be last point in they interplanar travelers.
Sorcerers? IMO in DS can only be combined with genies (form of genasi? IDK), but as a result - these people will be predisposed as elemental clergy. You have magical blood of genies, so you have to serve to elemental spirits. If not - your connections with elemental planes will be too weak, so death will be similar like a death by thirst. Or killed by some elementa spirit. Nobody (except SK and they templars) shouldn’t take super-natural energy of elemental planes without any services for elements.
Warlocks? Any non-wizard spellcaster in DS is de iure warlock Elemental priests have pact with elements. Druids with spirits of land. Templars with SK’s. If you broke pact - you can have big problems, similar like a warlocks in other settings.
Overall, I am in favor of Athas being strict and restrictive of “news from other settings”. Inspiration yes, but not copy-paste and change origin.