I've a strange take on psionic myself, since I don't like much the "innate power of the brain" approach. Psionics to me is just another way to use magic. But there's a twist on it.
IMC, there are deities who were ripped out of their body during a long-forgotten mythological cataclysm. They were reduced to simple mind substance (which is the basis of protoplasm). Lacking a body, they incarnated by reflex their mind into the very land itself. The event was so traumatic that most of them also entered a comatic-state.
So, you have massive divine consciences that are unconscious, and tied to geographic locale.
Psionics are people who learn to attune their own mind to echo the strange mindstate of these vegetative deities, and can thus grab a part of their divine power and command it, furnishing their own will to the Spirit's capacities.
In other words, it's an arcane way to use divine power. Few psionics know exactly how their powers works, and what they actually did. They just know that they have to modify their thought patterns, mindset, and emotions until they feel power under their command. They also know that different places will have different "aura", different emotions.
And now, of course, some of the Spirits are still conscious, and exerce their own will. Conscious places are usually extremely dangerous for psionics, since rather than controlling a parcel of a god-like being's mind, they risk having the being control their own mind.
By the way, the vegetative dreams and nightmare of these Spirits also explains lots of supernatural events, like the fiery storms of the Fire Desert, for example. Similarly, places where similar emotions are repeatedly powerfully felt will impregnate the Spirit and warp its mind toward that feeling. This can result in haunted places where hideous crimes have been comitted, that makes you feel frightened and depressed, or to the contrary, a hospital run by kind faithful of a benevolent healing deity will make you feel happy and relaxed.
With such a little twist on psionics, they can be integrated much more cohesively with a fantasy setting.
Even psionic combat can get its place, you're using the Spirit's wasted psyke to reach your opponent's one and turn it into the liking of some chosen portion of the traumatised godmind.