I recommend taking a look at encyclopaedia arcane: chaos magic by mongoose. It is just like what you've described, just a little over powered.
The only problem with the chaos magic stuff is you are going to die if you use it. put some balance on it and remove the eventual death stuff and you'd be ok.
Personally i'd look at the stuff from iron heroes, it'd need some tweaking but it's better.
I would very much suggest taking a look at Monte Cook's World of Darkness d20. It has a freeform magic system that is SPECTACULAR. It has feat trees for varying effects, rote spells and the ability to make just about anything you want magically without too much of a learning curve.
(I have not yet playtested it... this is an initial review after buying/looking over the class)
System wise, i like the medium base attack and d8 HD, though I could see it lowered to low BAB and d6HD if necessary. Skills are right on, though I could see them getting a few more specific knowledges of some kind depending on concept.
I cannot easily tell how the balance would work out for this class, but it definitely seems like a roleplay intensive class as well as something that is different, fun, and workable into the system. My notes/comments/questions are below. I definitely look forward to seeing it in action.
Spirit Companion - simple and easy, backed by the feel of the class.
Summon - i like it, but is there a maximum time you can spend on the summoning? When bonuses are involved, people will do some crazy things just to get bonuses...
Control Spirit -
Is there any more detail about what happens when the spirit possesses the shaman? I think there could be some really scary stuff outlined here...
Do you believe that 2xclass level is too much for feat simulation?
I'm glad you put the minimum caster level part in there... good cover. :)
Spiritual significance -
Why the cost difference for the magical item endowment? Why doesn't it count as actual magic for DR purposes? While I believe that it would work as is well, it does make it more difficult to keep track of. :)
Also, for the single person significance, is it too cheap? I'm not sure if it is or not yet.
Is spirit heal too big? 8d6 is a lot to START healing all of a sudden. Could this be an ability that was a bit earlier in the receiving but a bit less powerful along the way?
For lasting significance... Should there truly be XP involved if pathfinder doesn't use XP cost for magical items? If regular magic items use gold instead of XP now, should this? I can see the balance issues, of course, if it's all money, especially when the class doesn't focus on money anyway... I don't know how to balance that out.
Well done, and keep them coming. I'm definitely interested in seeing more. ;)