Ki seems a little cheesy as a power source to me, as does psionic - though it might be possible to justify the latter.
All of the other power sources seem to have ties to the cosmology of the world and its features. Arcane power comes from the Feywild, Divine from the Astral Sea (or at least the divine realms therein), Primal and Martial from the spiritual and physical makeup of the world itself respectively, Shadow from the Shadowfell, and Elemental from the Elemental Chaos.
I suppose Ki could be justified as a way of tapping into the powers of the Astral Sea, but that just seems like a tacky excuse for Monks and Ninja and Samurai to me. I thought the Monk class was a nice addition to the balance of 3rd edition but I never really liked the "martial arts" aesthetic for a more traditional medieval fantasy game.
Psionic powers could also be connected to the Far Realm I suppose, though I think the Far Realm should be kept as mysterious as possible, myself.