Some players will just flavour their spells as psionics and call it done.
Hear hear ! Flavouring is the way to play.
Or rather, in most cases, re-flavouring. Indeed, the D&D books always suggest some initial theming, flavouring, lore, or world-building. They certainly do so when it comes to what a Class or Subclass is (or can be). And especially when it comes to explaining why some characters can do magic. But nothing forces you to follow the suggestions.
As per the official suggestions, you could be casting magic spells, among other reasons, because :
- you studied the magic of the world, you understand how it works and it's a tool you can use ;
- it's innate to you, you "just do it" ;
- some Deity (or other ultra-powerful being) wants you to have spells ;
- you can feel the Force, I mean, feel the magic that flow through Nature, and tap into it ;
- you can feel the Force, I mean, feel the magic of the Word, and tap into it ;
- you understand the magic of Ki that flows through your body and can control it ;
- you believe in something big, you believe in yourself, and that magically empowers you.
Psionics is just another flavouring. It could be that you can cast spells because you have innate mind powers. Or because you studied, understand, and can use the power of the mind. Whatever.
From a mechanical standpoint, 5E already has various systems for spell-casting.
- Cleric and Druids have spell slots, a spell list, and prepared spells.
- Wizards work the same way, but not completely. They build their own spell list, instead of having a set one like Clerics and Druids.
- Sorcerers and Bards work with spell slots, like Clerics, Druids and Wizards. But they don't prepare spells (they know spells, as per the standard terminology). Or they always have the entirety of their spell list prepared, depending on how you want look at it.
- Warlock, well ... Some designers probably felt that there weren't enough systems for spell casting, so they made a new one. Warlocks know spells, much like Sorcerers and Bards. They have a resource called spell slots, which works like normal spells slots, except when it doesn't. Also, beyond spell level 5, they use yet another mechanics called mystic arcanum, and those are not spell slots at all.
I don't think that adding yet another mechanics for psionic magic would have been a great idea.
As far as how psionic powers/characters are represented in 5E, there are 3 Subclasses in Tasha's Cauldron Of Everything.
The
Psi Warrior Fighter and the
Soul Knife Rogue work essentially the same way as Battle Master Fighters. They gain a Subclass-specific resource call Psionic Energy Dice. They can spend those to fuel their special, psionic abilities. (The main difference is that Psionic Energy Dice recharge on Long Rests.)
The
Aberrant Mind Sorcerer works differently. It's mostly
psionic-themed (and pretty cool imo), but it doesn't get Psionic Energy Dice. I can't remember where and when, but I remember reading or listening to an interview from a WOTC designer (probably Jeremy Crawford) saying that since Sorcerers already have their own Sorcery Points resource, they decided against adding Psionic Energy Dice on top of it.
