centered set on the move..or a river?

We've noted centered sets with moving centers here and here, but what if the whole set moves?:

...I think that the centered set idea is closer to how things work in practice, but still wonder if the idea of the “center” in the “centered set” is too static. Is there one core of doctrines or beliefs we are moving toward or away from? Perhaps. Perhaps we could call it something like “Christianity,” but what would we put there?
I prefer a metaphor in which the whole continues to move. For a long time I used the metaphor of rays or trajectories. If two rays start off half a degree separate from each other (i.e., there is some theological diversity from the very beginning in Christianity) then if they continue on their trajectories they will get farther and farther apart. The result? An increasingly large space between the vectors that constitutes Christianity within the tradition; i.e., an increasingly theologically and culturally diverse Christianity that all can lay claim to orthodoxy.
If that doesn’t work for you, perhaps the metaphor of a river.

Image: prozac1 / FreeDigitalPhotos.net

This one I stole from Joel Green. The idea is that a river flows between banks. It is a dynamic thing. It probably gets wider and wider the farther downstream it goes. It is always moving toward its end. Moving together. It’s that sense of unified motion I like: even as it gets wider, the whole is still in motion in the same direction–toward the eschaton.
Or perhaps there’s a little of all of this. Perhaps there is a “center,” and that would be the Christ in whom we all live, and this Christ is not standing still or defined by or bounded by the creeds of the church, but continuing to march through history, gaining followers, and manifesting all the while an ever richer embodiment of the diversity of God’s creation.
 - J. R. Daniel Kirk