But if you look at the historical psion, "everything at once" is exactly what their niche was.
Sure, I get that. But that's an easy way to just not get a Psion class.
By slapping heavy restrictions on it you can add that part after the framework stands. You'll have a specialization in things you're particularly good at, there's no way around it. Anything outside the field you're specialized in costs the resource equivalent of a Spellslots 2 levels higher than the spell equivalent you want to access from outside your field of expertise. There you go. Want to be a psychic manipulator but still sling a Fireball? Sure, 5th level Spellslot for 3rd level effect please. Limited by your INT modifier/long rest. Or it's using up a Class Feature equivalent of Channel Divinity (Cleric) so no nigh infinte access to all low level spells when you need them.
A bunch of ways to make that possible by simply gimping progression and power in exchange for diversity.
That still leaves the question at what a Psion would be really good at compared to a Wizard, Cleric or Sorcerer.
I'd assume they start out with a different Spell list to access?
Options from every school, but mostly not the "best" option or lacking higher tier versions?
List of random ideas for variety with limitations:
- Healing is there but only single target, no reviving spells.
- Evocation but only "projectiles" that emit from the caster - Firebolt/Fireball/Magic Missle check, but no Toll the Dead.
- severly limited options for summoning (or their own uniquely flavoured options)
- I'd assume Illusion, Abjuration and Transmutation would be the focus. But could be a Spell level behind the other full casters, 6th-9th level Spells (not Spellslots) become available a level or two later.
- Not sure how to diversify a lvl 20 Psion and Wizard/Sorcerer, just not giving the Psion access toWish seems a little weaksauce.
As long as we can add the Psion's versatility in a gimped way so it becomes flavour/utility rather than added versatility and combat potential, this could work.