Gladiators. TFT is built on the chassis of a head-to-head, hex-and-chit mini-game. Gladiatorial-style combats are exactly where the combat system sings.
Preservers and Defilers: A perfect fit. The only difference between a preserver and a defiler is how you choose to act. All wizards can choose to draw magic from their own strength, as default TFT assumes. Or they can draw it from living things around them (saving their own strength). Make your choice. It's no wonder the templars are out to destroy not only renegade defilers but also preservers -- they're all a potential threat to the food supplies, making them an even greater threat to the rule of the local Sorcerer King than they are just for being wizards. They're not just competition. They're a potentially destabilizing force. Without any food, the desperate slaves may be pushed entirely over the brink into rebellion.
There are no gods. As in Dark Sun, there are no gods. Now, Dark Sun did give cleric players the option of being elemental clerics, but fuck that noise. With TFT, there are no gods, and that's that. Religious leaders can take the appropriate talents (Priest & Theologian). If they want to also true miracle workers, they can use wizardry to accomplish their miracles, always drawing from their own strength rather than defiling the world around them. As a bonus, summoning and controlling elemental powers is particularly interesting in TFT. Of course, that does make them charlatans (or deluded). Hey, it's a harsh world. Also, miracle working holy men and women are certain to attract the attention of the Sorcerer Kings's Templars, for all the reasons preservers and defilers do plus the fact that they're horning in on the King's whole "divinity" angle. Templars, of course, are just wizards with religious talents in the Sorcerer King's employ.
Races: The races are pretty well accounted for. Elves are already fast when unarmored (higher MA). Giants (renamed half-giants) are already a default player race. Even half-dwarves are mentioned in the monster/character section of In the Labyrinth. I'd probably want to add their exertion thing, but that's portable. Also, Hymenopterans are a good base for Thri-Kreen. Halflings, interestingly, have a bonus to reaction checks. I think this can be attributed to their tendency toward cultural mimicry in the Dark Sun setting -- they're trying so hard to fit in!
It's a dangerous world. TFT combat is brutal. Life is short and ugly under the dark sun. TFT keeps it that way.
But what about psionics?
Not built in, but easy to accommodate. Here's what I'd do. First, I'd pull out a list of "psionic" type spells from the default TFT spell list as available psionic abilities. Second, during character creation, anyone can learn these at no penalty as psionics. First psionic talent costs 1 IQ, second costs 2, third costs 3. (Probably -- or maybe 2,3,4). They can be cast at no DX penalty regardless of whether the character is a hero or wizard. Also, they don't require somatic/verbal components. However, unlike wizards, you can never draw energy for "casting" from the world around you (of course). Also, no spellbooks, item enchantments, etc. That's off the top of my head. There might be other balancing tweaks needed, but that's the general idea.