Hey look, I want "serious" and "robust" and "complete" psionics as much as the next guy...moreso, probably, than many since my homebrew world actually has psychics and psychic organizations in it...[thus why I don't just want to see "Psi in Dark Sun"]
But I don't really want, nor see a need for, a whole separate 1/2-PHB book of over 100 pages.
The "spells list" point is well taken, and sure, maybe we need 40-50 pages of psionic powers descriptions...assuming at least some of the powers will not be the same as existing spells...and in those cases, is not the point of the PHB and design philosophy of 5e (in general) to have that core "root" on which to "add to/build on?" I would EXPECT and WANT to see powers listed to say "As the X level Y-class spell. See page xxx in the PHB." rather than space and page count to just reprint the same description.
6 distinct classes? That's not really necessary or 5e's mode.
Let's take a walk through a hypothetical Psionics Manual, shall we? Let's.
Skip the covers. I'm going ot presume a soft-bound supplemental type manual. But they could go full hard-cover too. So, either way, you have 4 "pages" count there for inside/outside front and inside/outside rear cover. We're not going to be counting those. We're going to look at "content", so just keep those 4 additional plates in the back of your head.
6 pages for "white" pages, printing info, table of contents, etc...
4pp: What are [might be] Psionics? What might they be? Who has them? How can you use them in a game/campaign/setting?
NOTE: NONE OF THIS SECTION NEEDS/SHOULD BE PRESCRIPTIVE! Sorry branding managers. [No, I'm really not.] EVERYTHING in this section should be "
Is it...?" "These individuals
might be...?" "
Could Psionics be from...?" "Psychic powers
may be caused by..." NOTHING MORE!
I mean, come on. If I can come up with a half dozen possibilities in an afternoon, you guys ["professional game designers"] should be able to come up with at least that many to present without crucifying psionics with "One Origin to Brand Them All."
no more than 4 pp: Psionic Player Race options (whether innate or inclined towards). No more than 2 or 3 races or sub-races to existing PHB races. Maybe: Thri-Keen (throw Dark Sun a bone, as it is likely to be mentioned throughout this manual), Dwarf Subrace: Grey Dwarf, what were those variant humans, Ilumati or something? The guys with the glowing heads. Where were they from? But them. Maaaybe a Gold -oo! Or GEM!- Dragonborn variant...psionic in lieu of a breath weapon or whatever. That's it! That's plenty to add to an already nearing-bloated race list.
Don't need Maenads (nothing more than alt. humans), don't need Dromites (already have an insect race), don't need -what was that other ridiculous thing?-...oh yeah, "Half-Giants" (aside from an immersion breaking biological impossibility, where/how would having giant-blood make you psychically aware/powerful? Not a lot of psionic giants running around in D&D that I've ever seen/heard of. Just all "different for the sake of different" and/or "need something else so we can have another Large race"). They are all just corner-case/fringe bloat.
Not to mention, once Psionics are introduced into the system, it is trivially easy for 5e to ADD more races (or classes/subclasses), I'm thinking predominantly setting-specific material, at some later/other supplements or, as we do now, fan-creations.
6 pp: Psion/Psionicist: Telepath, Telekinetic, Seer, Metabolic, Porter.
3 pp: Fighter Subclass: Psychic Warrior. Monk Subclass: Soulknife. ...ok, fine, Cleric [or Bard? or Druid?!] Subclass/Domain: Ardent.
1-3 pages: Wild Talent Feat. Incorporating Wild/Random Psionics in your Campaign.
A dozen and a half pages, tops, on player class/race choices.
1 pg: Using Psionics in a "standand" game/combat.
1 pg: Psionic/Mental/"It's all in Your Head" Combat: How to play it. If you can't get this on 1 page, it has no business being in the game.
Options:
1 pg: Sorcerer Subclass Variant/Sidebar [1/2 page]: "Simple/Easy-Use Psionic" Sorcerer.
1/2 pg: Psionics & Multi-classing
4 pp: new Psionic Feats.
2-4 pp (total): Variant way of using psionics: PP, if that's not default, or "Slots", if that's not default. Psionics as ability/skill check? Variant way of playing Psi-Combat? Whatever it turns out to be, if you can't cover them in 1/2-1 page each, they don't need to be there.
Should be cover-able in 5pp: Discussion on the Planes. Ethereal, Astral, Far Realms. Psionic Travel. "Psychic Combat" on. Psionic risks of.
Sidebar/Alt. Planar Variant: Plane of Dreams.
Let's say, for arguments' sake, we have -at this point here- roughly 32 pages. Following what we've seen of the design/layout of 5e books thus far, we can pretty much guarantee an additional 16 pages worth of art and layout/white space. That brings up to a lovely little 48 pp manual. This is, of course, the most malleable area for spacing.
Now, powers, as noted...let's be generous and say 50 pp will be necessary, though I struggle to see how that would be possible unless you ARE reprinting all of the psionic powers-with-spell-equivalents...but be that as it may. We'll just say 50 (we could take this to include art, as well).
Still brings us in, just under/kissing 100 pp. Now, obviously, if they go one direction or another on the amount of art [presuming layout will be in keeping with the rest of the line] and this could be moderately to significantly closer to 75-80...or could expand well above 100 [with primarily "wasted"/padded space, imo].
SO, yeah...a "not more than 1/3 of the PHB/DMG" is more than sufficient, imho, to get a solid "Bringing Psionics into 5e" manual in our hands, with significant and usable options -a smattering of races and enough classes to cover the major archetypes- without going all bloaty/crazy/"everything psi that ever was."
Is there anything seriously "pressing", that I missed/didn't remember, for introducing/including psionics ...into 5e in a usable form? [and this last is crucial to remember/think instead of just, "But I like/want X from Y edition, cuz mememe!" Does this give you
enough to get you rolling with psionics in your 5e game?]