Well, these are cool. I'm not that familiar with Dune.
It seems there are some similarities but superficial ones?
For example, Dark Sun has their assassin orders, secret societies, guilds, etc. - each with their own commodities I think. But maybe the world doesn't reflect the need for these beings to exist.
I think it is interesting to compare these settings.
On retrospect I think I should expand a bit on my view that the similarities are superficial. When I say that, I'm talking about the whole scope of Dune vs. Darksun. But Athas and Arrakis have a more similarities partially due to the needs of their respective biomes which their inherent cultures are set in.
So, how would a Dark Sun bards (spies/negotiators/assassins) look like if they were Bene Gesserit? Maybe they would be more religious and respected?
If we were going to use Darksun as a template (i.e. D&D class-based mechanics), while thematically it would be close. Bene Gesserit would be probably be closer to being Monks with some mystical perceptive abilities tossed in. Ironically you could also do this with Psionic classes via 3.x
What about Psions? How would they differ from mentats etc?
"Mental powers are used on an everyday basis in Athasian culture.
Mentats are super-skill-monkeys in Dune. They're capable of *fantastic* cognitive levels that could replicated mechanically as psionic/magic powers. Clairvoyance/Audience, Augery, etc. on top of very high mental stats. By going full Psion, I'd probably give them a specific range of powers to choose from. The caveat here is that the more powerful you allow your psionic/magic systems to be, the harder you'll have to be on controlling the proliferation of such powers. The game could easily have recommendations for such power/control sliders. Or just go wheels off. There's a LOT of powers that Bene Gesserit/Mentat/Guildsmen would have that overlap. But there are some that are unique to each faction/class. Bene Gesserit definitely have Suggestion and at higher levels Charm - one might argue Dominate. Guildsmen may have Blink, Teleport and other space-rending/perceptual abilities.
An important distinction would be to illustrate the customs of each faction and *why* they do what they do - and why their respective powers are secret or largely occluded from the others despite their sources being similar. This implies the training techniques would be so arduous and intensive that either it affects their worldview or their respective sources are fundamentally different - The Mentats use Sapho while the Bene Gesserit use Melange. Coupled with their distinctive training, they produce different results (be it class, skills, feats, edges, etc)
Athas has its religious cults around the Dragon Kings (Templars) and would be closer to Dune, post-Paul. Athas largely ignores the world outside its desert environs but the conceits are closer to God Emperor, only you have a bunch of mini-God Emperors each playing chess against one another. Whereas in the Dune scenario you have many powerful factions playing chess against the singular overlord.
Ideally were I to do this setting - I'd set it up pre-God Emperor and have the rules allow for a PC to potentially take that role (but not necessarily mandate it). Call it the egalitarian streak in me, the competitive challenge of designing fiefdom-rules to contain enough heft to allow a PC become the central figure in their world's history is pretty tantalizing. The potential PvP would be very high and might necessitate NPC status for the new God Emperor as the campaign shifts into a generational conflict pitting the PC's against their ancestors. Yeah... that would be daunting and fun.
Telepaths allow instantaneous communication across hundreds of miles. Draft animals and slaves are kept under control by psionic overseers. Prophets use their visionary powers to forecast the fortunes of kings and peasants, find missing objects, and solve crimes. Kineticists and egoists use their potent abilities in all manner of enterprises, both legitimate and otherwise. "
So maybe in a Dune-like setting they would have a more pronounced social role instead of individual abilities?
This is the nuts and bolts of designing these conceits into the game and how they would be self-regulated. Guildsmen might have telepathic abilities to communicate over vast distances *in order* to focus their teleportation or navigation abilities. Prophets and ascetics of the Sisterhood advice kings, royalty, personages of power, while carrying on with their Breeding Program to produce their vision of the Messiah, the perfect Bene Gesserit that is male (or whatever cosmic conspiracy floats your boat). Another big aspect is considering what emergent abilities will manifest as time wears on - including itemization- which is also a big part of Dune. In a fantasy version it would be even more pronounced.