In a perfect world with FFG published programs to aid in character creation and play each power could rely upon a mean of Willpower and whatever characteristic was most appropriate. I want to Shape Flesh? Roll against Willpower + Toughness / 2. One can dream.
At any rate, something I think would be lore friendly but perhaps meet a lot of rejection here would be psykers starting with all the powers in their discipline. However , the powers would need to scale, somehow, perhaps with psy rating, to represent weak psykers being ineffectual at the more grandiose uses of their warp manipulation
I say this because in every instance of a Black Library book I've read that has psykers in it they don't use one thing over and over again. Most of the time they seem to have control over some "field" of things that they use differently depending on the situation. Eisenhorn used telepathy to alter and strengthen his interrogation, pick up on people hiding things, force foes to make mistakes in combat. Ravenor was crazy strong and used divination and telepathy in more ways than I could explain here. Patience Kys was a telekine who used her abilities in a multitude of different ways.
Having psykers have "powers" that they have to purchase, while much easier to write, feels incredibly lore-unfriendly to me.