Wizards has updated the playtest document for the 5e Mystic class. Take a look at it here.
The class now goes to 10th level. The core concepts had enough support that we feel confident moving forward with them.
Psionics is more flexible. You have a psychic focus, which allows you to pick one discipline and gain a constant, special benefit from it. Otherwise, you can expend points to use any discipline you know.
Psionics now includes psionic talents, the equivalent of a spellcasters cantrips.
Psionic disciplines are now available to all mystics, regardless of mystic order. However, your order grants you a benefit for using its associated disciplines.
Psionics have always been a part of the Dark Sun setting. Within the cultures of Athas psionics are referred to as the Way of the Unseen. They are a utilization of inner power, a blending of the physical and spiritual energies to create fantastic effects.
Here are a few thoughts on what psionic pieces from previous editions should included in 5th edition D&D.
In the ancient days of Athas' history those who used the Way could travel swiftly from one city to another using powerful psionic Markstones. Today some of these psionic artifacts still exist, allowing those who have the Will to travel faster than the fastest elf.