It’s amazing, at times, how an archetype can be represented in the rules. Case in point, I was recently looking at converting the mystic to the 4th edition D&D rules. Easy peasy, right? Not so much.
Class creation in 4th edition is far more difficult than in prior editions. With thirty levels of powers to create, it’s incredibly difficult. Plus you have to consider paragon paths and epic destinies. No thanks.
The mystic was a “replacement cleric” anyway, so why not use that? Or what about the warpriest build in Essentials? Sure, that would work, and the warpriest would give you the single domain the 3.5 mystic had. But, the downside is that mystics are not set apart from clerics. There’s nothing mechanically different about them.
How about psionics? While the SAGA-era mystic had a lot of psionic crossover, the fact remains that many Dragonlance fans have a stigma against the use of psionics in the setting. Perhaps its due to the sci-fi feel of psionics from prior editions. Plus, there are continuity issues. If continuity doesn’t bother you, though, psionics may be a good fit, in particular the ardent.
What about themes, ala Dark Sun? That would definitely be a good way of portraying any sort of mystic you like. Yet for those wanting a full class, they may find this option lacking.
The fact is, you could use any of these ideas and more, and find that in each one there are perks and flaws. The key is in finding the option that best fits your character idea. From there, feel free to re-skin the class and power fluff to match the type of character that you want to portray.
While I hope that we will see a mystic class someday, it is good to know that we, as players, have options. Find the one that best fits your mystic and run with it.