I don't fully know how to handle the spirits issue, but I think the idea is that since the Chosen of Air would be picked from the newly-formed Air Nation, the Chosen of Air would try to act as the Spirit liaison for the Chosen, if no one else could handle the duty (The Chosen of Water also sounds like an optional liaison, what with the whole spiritual waterbending deal). And besides, the Big Bads so far are very human people, who don't go out of their way to bother the immortal, indestructible spirits.

There is also only one non-bender spot available, the Chosen of Man, and that's it for the main cast before I even consider supporting/side characters.