The shardminds would be "broken" or OP (overpowered) in Dark Sun because they are living constructs and then they don't need food or drink. My suggestion would be in the Athasian Tablelands the living constructs PCs can be allowed but to recover the power balance then we add something like an elemental fungus, something style "red weed" from George Orwell's "War of Worlds", a disease for constructs, created as a biological weapon. Then to stop the infection the living constructs need "medicines", simple biological fresh tissues.
WotC has learnt with the experience sourcebooks about alternate powers aren't sold so good, and this is worse in the transtaltions to other languages. And a second book about those alternate powers are harder to be sold.
I would love a 5e version of Tome of Magic and Tome of Battle, with more classes, but even if this happens some day, we will have to await a long time.
I loved the martial adepts, but the gameplay was opposite to the current phylosophy of the 5e where the rules have to be simple and easy to be understand, and combats fast. The classes from "Adventures of Rokugan" (bushi, shinobi, duelist) have got something like this, but I need more time to understand it.
I guess WotC wants the psionic mystic class to be something like the D&D version of the "cultivators" from
xianxia fiction.
If the duergas are added to the PH, I guess it will be as subrace in the same level than drows.
Some times I miss the spriggans, the evil cousins of the gnomes.
* I have written "psionic" for 5e in DMGuild and there are 457 articles (incarnum only 9). This should mean a group of players are really interested into the return of psionic. Of course it would need a lot of playtesting and feedback.
Some times 3PPs have got some good ideas, for example the Athanatism, the psionic discipline about the manipulation of souls and spirits. (Dreamscarred Press).