Zardnaar
Legend
Like most have said, I suspect artificer will make it into the WGoE, but psionics will wait until Dark Sun. Eberron has a place for psionics, but psionics isn't integral to the setting.
That said, I very much suspect the next setting to get a PDF will be Dark Sun, and I suspect it will look very similar in execution: A general overview of the setting, few new races (muls, thri-kreen, etc) and overview of the PHB stuff and how to fit it in Athas if you want, the Psion/Psionicist/Mystic/Fnarg class and any psionic subclasses, an updated equipment list for weapons/armor, and an overview on defiling. They will leave a fair amount of details (like classes) intentionally vague so that DMs can decide if they want the arcane bards of 3e/4e of the assassin nonmagical bards of 2e. I also would wager we see the first version of the Templar's Guide to Dark Sun sometime in late 2019 or 2020.
I would bet on Wanderers Guide to Darksun or Wanderers Guide to Athas (I like this one).