Back when I played a druid for a session, his Wildshape form was an earth-elemental lookin' guy. Same deal.Well their not even specifically animal spirits, by the official fluff their just "primal spirits of nature". So they could really look like anything you want, and if your fine with not going with the official fluff I think a spirits of the dead could work just as fine.
True, I just see Spirits as more... well, important, so why is one fighting for a shaman, by the shaman's direction? It's almost a subserviant role.I am viewing it more that the will of the Shaman helps to reinforce the spirit into being connected to the world/material realm.
But, the more that I think about it, I can see the spirit basically being something that either:
1) a low level spirit assigned to the Shaman by a powerful spirit as sort've a protector ("Ensure this speaker's safety").
2) A low level spirit seeking to grow, so it works with a Shaman in order to grow in power/influence. This means a lot of grunt work, but it's getting rewards too.
3) The Shaman's soul. The spirit companion is actually their own self on the Spirit Realm, and they can bring it forth.
4) Other spirits. Rather than one that sticks with the shaman leveles 1-30, it's more like a temp position for any local entity that the shaman can lobby for whatever. Or it could just summon 'anyone that answers my call' with that minor action.