The oriental view of these gods is not as being omnipotent and perfect. Infact Monkey and Co in the series are largely quite the oppersite, more like the heroes of mythology.
Given the nature of D20 and the fact that most fantasy games the players characters are like gods to the average bloke in the street, why not offer the chance to play one of the more minor spirits (Horse, Cow, Raven)...
As for character classes, well theres a warrior, Mage, Monk, Clerics, Sorceerers, Thieves, Assassins, other spirit warriors... I think we can see where this is going... The main difference is that many of those in the series have a bit of magic in them (Slaine also has this)... The idea, as mentioned before, is not to actually just have Monkey, but a mythological game of the orient (like D&D essentially is just Lord of the Rings)
I'd see it the other way, the monk is more likely GM controlled (its a perfect being remember, a Buddha) where as the others are largely drunkards, fools and braggarts (your average pcs). Me, I'd turn all four over to the players, and let them roleplay - after all thats what its about.
Just because its called Monkey, doesn't mean that YOU are playing the central characters (see all licenced games for this - rather you are playing in the world they inhabit). In Judge Dredd you don't actually play Dredd - and thank god, because I play in a Buffy game....