And then I started pondering the big picture- do I really want psionics in my Dungeons and Dragons?
I will preface this by saying that I love psionics.
Then the answer is 'yes,' no need to over-think it. If there isn't a psionics system in the current ed (yet) you can adapt something from a prior ed or come up with something, or re-skin something (GOO Warlocks seemed like a good candidate, to me, in pre-Mystic 5e) until they get around to it. If they finally come out with something and it's not so great, fix it up to your liking.
So I devised a new set of rules, incorporating some stuff from the appendix and a bunch of homebrew based on some books by Julian May, and that worked out well.
Many-Colored Land? With the torcs and everything? Yeah, that was almost written to be used in an RPG, quite a structured and defined take.
How does a good psionics system work in D&D?
That's a trick question. If a game has any 'good' systems, it's not really D&D. ;P
Pros:
1. Psionics are cool. We like cool things. Really. This is the most important point in favor of psionics, and it shouldn't be discounted.
2. Intelligence matters. The best, and pretty much only, gameplay reason is because intelligence is, perhaps, the most devalued stat in 5e, and psionics could make intelligence great again.
3. Variety. Variety is the spice of life, and all that.
4. History. We have had psionics in D&D, so, you know, why not?
Of these, honestly, (4) is valid in the context of 5e, and some of (1), as far as the bit about not discounting flavor (that is, differentiating things that are only different in fluff/flavor with arbitrarily distinct mechanics).
(3) Meh, re-stating (1).
(2) Arbitrarily using a given stat as a given class's primary does not make the stat suddenly relevant to everyone. INT is already the Wizard's primary stat, CHA is already primary for several classes. That doesn't make them valued, what makes stats valued is uses everyone puts them too - skills, initiative, saves, AC, hps, etc...
Cons:
1. Not part of the core rules. It's really, really hard to retcon a new system on to the core rules and get it rights. Stuffing something into an appendix (ahem) doesn't mean that future published products will take it into account.
2. Implementation. This is the big one, and I will explain why I think this is the big problem below.
(1) should be a non-issue in 5e, as it's meant to be modular. OK, it's not modular for any definition I'd accept, but I'm used to use of modular in programming, and that's perhaps overly stringent for the context. Anyway, 5e is meant to be a starting point you can add to, it has lots of stuff you can add (including Feats & MCing rules right in PH). Being left out of 'core' should in no least remotest way disqualify anything from being added later.
(2) Yeah, it should get as good an implementation as WotC can manage with the resources they've devoted to D&D development. But it doesn't have to be perfect, and it's gauranteed not everyone will like it, and thus, many will tweak, ban, change, mod, or totally re-write it. That's how 5e's meant to be (ab)used.
a. First, you have to decide whether it is a class-based system or not.
Not really. 5e is a class-based game. It'll be one or more classes. 5e also lets you poach from classes via backgrounds and feats. There should certainly be a 'Wild Talent' background, for instance, swiping a bit of Mystic mojo the way Acolytes dabble in Cleric stuff.
b. Once you decide to go on a class-based system, you end up with something like the Mystic. But here's the thing. 5e is already built on a spell basis. Really. Almost every class either casts spells or has spell-like equivalents.
Every character class, even the Barbarian has a build that uses a few spells as rituals.
Most monster abilities are written in terms of spell equivalents, and magic items as well. So ... you end up having a mystic, in effect, "cast spells" using their mind. You can fancy it up (they use "psi points") but you still have to call it magic (as the mystic does) in order to incorporate it into the system (dispel magic, etc.).
Or the DM can decide it's different and doesn't get dispelled (nor dispel) magic.
So psionics is basically just a fancy magic system by another name. And then ... what's the point?
Same point as the Bard, Sorcerer, Warlock, & Druid. To be, per 'pro' (1), above, cool & fluff-distinctive. So psionics might reference spell descriptions in using their spell-point-like 'psi points?' So what?
Do you want psionics in your D&D?
Personally, whatever system they come up with, I likely won't opt into it. Too sci-fi for my taste.