Here's what I'm thinking...after a half dozen (at least) "Psionics, in 5e, How/What/Where" threads:
1) Psion/Psychic [full/base] Class.
Has its own mechanic for manifesting powers be this borrowing from Sorcerer or Ki Points, Warlock Invocations, ability checks for sustaining (I wouldn't do it for initiating, but maintaining) powers, recharging on short rests, whatever, and/or any combination thereof that is beyond/not the same as existing "casting" mechanics/no "spell slots".
Opened Mind: A list of low level/power general "psychic combat" [offenses & defenses] and detection [detect sentient mind, detect good/evil, etc...] powers that any/all psychics receive. For example, something akin to "Detect Psionics" or a "Danger Sense" (Something "feels" wrong) would be ok, something like Mage Hand, Detect Thoughts or Clairvoyance would be too specific/belong among the subclass powers.
Proficiencies: Light armor, simple weapons,...set of tools of choice?
Subclasses of the Psion/Psychic Class: Telepath, Telekinetic ["Teke"], Seer [Clairsentience. Separate power lists that the subclass gains access to at this choice point (2nd or 3rd level, as best matches other "full caster" classes).
So there's -at least a start- your 2-3eish major disciplines. To my mind Psychometabolism and Psychoportation can definitely take a back seat (or really go away entirely, afaiac), are already done in a variety of ways in the system and/or belong, more appropriately, to other classes (like Druid Wildshape, DimensionDoor/Teleport spells for mages, etc...). Trying to force them into a Psionic introduction is both cluttering and adding unnecessary complexity. You want to have "See these are MIND powers, /not/ the same as Magic", so make/show them MIND powers. 3 subclass/disciplines of the most common avenues of "psychic powers" is more than sufficient for introducing psionics to 5e.
2) Subclass of Fighter base class: Psychic Warrior. Maybe with powers introduced in a "Domain Spells" kind of way [for everyone who wants to play a Jedi in D&D fantasy.] Roll in "Soulknife" or whatever it was with the "psychic blades" as a higher level feature.
So there's your Psychic Warrior and Soulknife that lots of folks seem to want/think is what Psionics is. And a footprint to follow if you want to do your own "Psionic Existing-Base class."
3) Optional/Variant/Sidebar: Wild Talent Feat
Get whatever the bare minimum of the manifesting mechanic is, be that minimal points or a once per short rest or what have you. Select from a list of powers that you use at whatever the manifesting mechanic allows. Maybe, to borrow the structure used for existing optional feats, choose 2 "General Psychic Powers" and 1 talent/gift from the Telepathy, Telekinetics or Clairvoyance power lists of the lowest "tier/point cost."
There's your 1e, random psionic power on top of any PC of any class.
So I think that works. All this arguing over "no do it this way, no do it that, no its distinct enough for its own class, no it's fine as a sorcerer variant, no it should be a feat"...Just hit it full on, introducing every possible option: full class, subclass to existing class, feat. Done.
No, you're not getting your separate psionic rogue and separate psionic monk right away, but the example/ground work is there, the precedent has been set. "You /can do/ a psionic X class! See? Like they did with the Psychic Warrior."
That's all that's needed for the introduction of Psionics to 5e.
Now, this business about "how do we flavor it/should we change the flavor [since we're being told to do something that has to be branded as 'D&D psionics']" is a whole different question/story. But, since fluff/flavor is stripped/ignored from the 5e system as easily as breathing, I suppose it's really a non-issue.