There is a huge elephant, or possibly mekillot, in the room when it comes to the mystic and psionics.
Psionics in D&D is primarily associated with two settings: Dark Sun and Eberron. In Dark Sun it is a defining part of the setting, and in Eberron it's an important thing primarily connected to a PC race and a particular villain faction (which means it's in a place where the DM can make psionics a big part of their campaign, but if they don't like psionics they can pretty much ignore it). But here's where it gets dicey.
Dark Sun is built around the Complete Psionics Handbook for AD&D 2e. These have a fairly flat power level (high-level psionicists primarily get a larger arsenal of powers, but not very many more powerful abilities), and focus pretty heavily on the telepathic, clairsentient, and to some extent psychometabolic parts. Yes, telekinesis exists, but while psionic telekinesis is available at far lower levels than magic TK, it's also far less powerful. And psychoportation is nice and occasionally a bit powerful, but it works better when used in support of other abilities. Psionic combat is a big deal, with special rules for various attack and defense modes (with more detail added in The Will & The Way). Psionics are subtle (no tell-tale signs), but limited (each active power drains psionic energy from the user).
The psionic parts of Eberron, on the other hand, are built around the Expanded Psionics Handbook for D&D 3.5. This book makes a strong attempt at equalizing psionics and arcane magic - a kineticist can throw out some seriously heavy-duty energy damage with unmatched flexibility, metacreation can create astral constructs that are at least on par with the meatbags a druid can bring into play with summon nature's ally, and all psionic disciplines have some way of dealing heavy-duty damage. Psionic combat is de-emphasized, perhaps due to the poor way it worked in the 3.0 psionics rules. Psionics are less subtle with almost all powers having some kind of tell - glowing eyes, ectoplasm, or whatever, but they can be very powerful, particularly at higher levels.
Creating a set of psionic rules that work well for both Dark Sun and Eberron... well, that's not going to be easy.