Incarnum had flavour?
In any case, it seems to me that the first thing to do if you want to show that the concept of ki isn't necessarily tied to Asian trappings is to stop using a real world Asian word for it.
About what I was going to say, so I'm glad I waited to get to the end of the thread to say it.
Ki (Japanese, coming from chi in Chinese) means "power" more or less; it's the same character used to mean healthy or electricity, and has nothing to do with the soul. Saying that your power source is energy is about as bad as saying your power source is power. Of course, this is an "Asian" word for power, so perhaps this suggests that for ki users, their power source is Asian power?
If we take a step back and look, we can see that Martial power source comes from the body, and the Psionic power source comes from the mind. Ki could, arguable, come from either - since both are already taken, though, it doesn't leave much room for ki. Other than, of course, as the resting place of all classes Asian. Other than being a bit silly, this also means that many DMs might think that ki has no place in their campaign, and simply disallow all associated classes.
I appreciate the argument made in regard to a power source inspiring new classes, since the Warden is probably the result of this process and is awesome. However, I think that any non-Asian ki class you could think of could probably appear under Psionics instead. Ki would only inspire more Asian classes, like the samurai as a ki defender. And really, the samurai is nothing more than a fighter with a different set of clothes.
That said, I disagree completely with whoever claimed that a ninja is exactly a rogue. The ninja, especially in modern parlance, is strongly associated with supernatural powers (as in, does not belong in the martial classes), and a distinct fighting style and flavour. Though I tend to dislike the idea of including classes with "Asian" (generally Japanese) names (like the aforementioned samurai) I think the ninja is now common enough in western culture to appear beside wizards and clerics.