... What power source do they draw on? Because "the mind" does not seem like enough. We don't have psionics in the real world (as far as we know) because of conservation of energy, you can't have events and state changes that spontaneously happen without some source of energy. That energy has to come from somewhere... Because saying that you reject reality and substitute your own may work for an alien mindset, I just don't see how you differentiate it in practical terms.
Reread parts of this thread. This concern ahas been addressed over and over and over and over and over and over and over.
When psionics has worked well in my games, it is evocative of comic book super heroes. That is extremely different than a wizard, sorcerer or bard.
Mechanically: Your PC has a limited set of powers, and they can decide on how powerfully they want those powers to operate. They can drop a lot into a big bang, or they can use a little power at a time. This gives them the ability to do the dramatic 'impossibly cool' thing by concetrating their remaining power into one big event. While some spellcasters have a limited set of powers, they can typically take very different powers - while the psionic character is tied to a thematic core. They have a narrower 'skill set'.
Lore wise: This is different between campaigns, but in mine there is a huge significance to psionics being "the" power source that does not originate from the spell weave. Arcane casters steal power from the weave. Divine casters are delivered magic through the weave. Nature casters pull magic through the weave from the positive and negative energy planes .... but psions train their souls to be power generators. They are their own power source, and thus their psionic abilities are not impacted by magics that deal with the weave such as dispel magic, detect magic and counterspell.
Further, psionics were unknown before the Far Realm collided with the Known Reality. When this collision took place, Devils were corrupted into Demons; and the Prime Reality shattered and broke off the Shadowfell, Feywild, and Ethereal Plane (three reflective planes that are half Prime Material Plane, half Negative Energy Plane/Positive Energy Plane/Far Realms). At this time, when Aberrations began to force their way into reality, the psionic powers of the Far Realm also began to appear and people began to learn to use them in ways that were previously unknown - from forming the first Monastic Orders to learning how to perform psionic miracles without magic.
If you try to implement that idea by just reskinning a wizard or sorcerer you lose
a lot. I've tried it. Many times. It sucks. It feels contrived. It gives you that boring feel that so many people complain about in this thread. The difference between me and those people is that I know you can do more with psionics to make it feel distinct.
Will it have some common concepts with spellcasting? Of course. It will also have things in common with the Battlemaster Fighter like attack rolls and forcing opponents to make saving throws. However, if executed well, it will not feel like a fighter, wizard, sorcerer, bard or any other existing class - in my model, it will feel like a comic book super hero inserted into the game.