Which is transformational. It changes their nature. They were strictly martial, they become casters.
"Add to" is a change. I get that you're fixated on the kind of change, but there is precedent for sub-classes changing the base class in dramatic ways. Adding spell casting, for instance is radically changing spell access & spell lists.
You and I are using "change" very differently. For you, any deviation from the "norm" is change, whereas I'm solely focused on contradicting what came before. There is nothing about EK's spellcasting that contradict's what the base fighter does. Both still wear heavy armor, use weapons, can heal self a bit, and can occasionally go nova with extra actions.
There is no subclass that changes a base class feature. None. A few add extra options (like extra uses for inspiration dice or more action choices in cunning action) but none of them remove spell access, change the spells they have access to (except to add more options), remove proficiencies, or change arcane to divine or vice-versa. You are asking for a rules expansion equal to or greater than adding a new base class; you are asking for subclasses to do more than they currently do.
And Sorcerers, Warlocks, and Paladins CLERICS do so at 1st. The thing about a precedent is that it doesn't have to happen the majority of the time to be established, it only has to happen once. Three classes picking sub-classes at first level is more than adequate precedent.
For those three classes only. No other class does. Doing so creates a very odd rules position: if a psionic subclass has to be picked at first level, what about others? Do all bards now have to declare being Lore, Valor, or Ardent at first level? What if you don't pick it a first level, can you now NEVER select it when you get to third? Why does the psionic bard change my proficiencies at first level, but the Valor bard doesn't? What if I want to mix classic base bard abilities (like musical instrument proficiency) with the ardent bard psionic powers? What if I choose psionic bard at first level and change my mind when I get to third level? Can I still take Valor or Lore now?
Your creating a recipe for headache, and since WotC is very keen on keeping this game AL compatible, I doubt they'd create something that causes THAT much chaos.
There are, in fact, no sub-class rules in the PH. It's not a class-design ruleset, it's just a set of classes. What there are, is precedents, in the existing 38 sub-classes. [/QUOTE]
That's ALL we have. There is no rules for character class creation either, but design a class with full-spellcasting, d12 HD, double proficiency bonus to all class skills and Con/Dex/Wis as proficient saves and tell me your setting a new precedent, and you'll be laughed off this board.
I shouldn't have to explain why adding a sub-class is simpler than adding a new base class, you've been pointing it out, yourself: most of the class is already designed, you just have to design what makes the sub-class unique.
What makes a subclass unique is a.) additional powers given at specific levels, b.) additional proficiencies given when the subclass is first taken, and c.) possible additional spells added to spell list. All 38 subclasses in the PHB adhere to this. Both of them in the DMG adhere to this, and the four in the UA documents adhere to this.
A Psion as Sorcerer sub-class, for instance, is easy. The Sorcerer even already uses a point mechanic. Your heritage is Psionic (maybe Far Realms if that's used explicitly), you get some features related to that, like telepathy and psionic combat, say, and you're off.
Which makes you a mage with a few psionic powers. Good job. You're a fireballing, magic missiling, mage armored, meteor swarming spellcaster, who gets a few telepathic or telekinetic powers at 1st, 6th, 14th and 18th level. Iits the definition of "we don't care about psionics, so here is a sublcass we threw together during lunch break. All your Dark Sun or Eberron psions and the like are just sorcerers now. Have a nice day."
If WotC is going to slap psionics fans in the face with "just play a sorcerer" I'd really rather they not bother. I can make my own subclass if I want that. I'd almost be happier with "psionics is dumb, no psi in 5e." than this half-baked option.
Depends on what goal of the design is. A balanced class shouldn't break anything, unless there's an unintended/undetected synergy with some other game element. A class designed, as most 5e classes have been, to evoke classic feel, OTOH, may be a tad 'broken' by some measures, if the original - like the 2e psion, for instance - was.
There are lots of fixes you could add to 2e psionics to keep the better balanced; minimum levels on powers (no disintergrate at 6th level); max cap on power points spent on a given power (no nova big blasts), some general magical overlap (at least as far as detection and dispelling is concerned). I played with both versions of 2e psionics; they were so close to being good, but some minor things allowed them to break. One hopes 5e's design team could iron out those kinks while keeping the general tone and feel of psionics similar.
I'm fine with new mechanics, if they're clear/balanced/playable mechanics (which isn't even relevant, since obtuse/broken/problematic mechanics are hardly anathema to the 5e philosophy). But, new mechanics present challenges. It's not a catch-22, it's a trade-off. There's greater design effort and you're 'betting' more if you go with the more extensive splatbook vs a UA article, for instance. Both are valid options.
I fully expect psionics will appear in a UA first. A decent psionic class shouldn't be more than a dozen pages to start. We'll play with them and break them and then they will fix them and release them in some finished form later. Its WotCs modus operandi these days.
Sorcerer is the most obvious choice for a psionic sub-class, it's been brought up in multiple threads here and on the WotC boards, and it's pretty intuitive, being the class for in-born magic. In addition, Barbarian, Fighter, and Rogue have no magic abilities to 'change' before choosing a sub-class, so tacking on psionics to them the same way arcane is tacked on with the EK and AT or rituals tacked on to the Totem Barbarian would be fine. Not that I can think of any reason to have a psionic barbarian.
Psychic Warrior can bolt on to Fighter like EK does, not problems there. Likewise, a Lurk/Soul knife seems keen for rogues. Add on a dedicated psion class and a "Wild Talent" feat that mimics Magical Initiate, and you have a well rounded psionic stable.