I would run psionics on ki, and each psionic class would have it's own ki abilities baked in, and there would be feats that just give you ki and some ki features, as well as subclasses boosting base ki features or giving you new ones. Most ki classes wouldn't have spell level type stuff, but instead be level gated like most monk features. You get access at a given level, it has a certain cost, it does a specific thing, that's it.
A Mind Mage or Psion or whatever would have telepathy, some telekenetic abilities like an invisible mage hand and the ability to throw stuff, etc, and those abilities would get more powerful as you level, eg "starting at X level, when you use the Move Object ki feature, you can move an object of up to huge size, and your telekenetic damage die increases to 1d8." Such abilities would be fairly open ended, playing more like a Jedi than like a wizard.