I have to agree with just about all of what has been said in this discussion. I personally perfer to see him as the Guardian of Change because of the obvious reasons as well as what was seen in the second movie but also for another reason.
I like to see him as the representation of change because I like to use The Big Five or whatever you perfer to be a bridge between children and adolescence. Mainly because when I write something about them I don't have to censor some of the events just because it's leaned more towards children.
If anything Trust, Empathy, Change and all of the other ideas that have been tossed around could just be what lead to his center and it can differ from person to person.
If only North had made a Russian doll of all the characters so we wouldn't have to debate what their centers are ha ha.