In addition to the above, it makes sense to describe how Ki will work as a power source.
The first concept to tackle is to define what Ki is as a power source. You want to make it different from Divine, Arcane, and Martial, yet still recognizeable as a power source. Ki is often described as the active principle forming part of any living thing (wikipedia). It is also understood as the 'breath' or 'spirit' of a living being. It is associated with the 'mind', the 'heart', 'feeling', and 'the atmosphere'. Ki 'vapors' arise from a corpse after death, demonstrating the dead person's ascension to the beyond. In traditional Chinese medicine, Ki travels through the body in natural channels known as 'meridians'. Having a blocked channel or an imbalance in the flow of Ki can cause disease (there is also a case to be made that poison is anti-Ki in a sense, and when introduced into the body, block or damages the flow of Ki). This all sounds cool, but it needs to be boiled down into a concrete concept that can be made into a D&D power source. My assertion is that Ki powers affect living beings, and usually comes from an internal source. Does that mean Ki powers can't affect objects or the undead? No, because that would be against the basic tenets of 4th ed. But I think Ki as a power source should be primarily internal, and primarily affect living beings.
The second concept to tackle is how the power source affects the classes' powers. Martial seems to use weapons (whether melee or ranged) to 'project' the powers. Divine and Arcane have more area and ranged powers and have few melee attacks, although you could argue that Paladins and Swordmages (FR) are Defenders and use almost exclusively melee powers. I'd suggest that Divine and Arcane tend to project energy type effects much more than standard untyped damage (such as that of the Martial power sources). How does the concept of Ki fit in with a power source, and how will that affect powers and how they are projected?