My BC experience is limited, but I like your idea.
Let's see, what does Undivided do right now?
Re-roll on your mutations and pick the best result.
Allied with everything for skills/stats
No rewards/no marks.
Maybe dedicating yourself to being Undivided could be a Talent? It would let you select a few skills/talents as unaligned instead of aligned, and maybe give some sort of fellowship bonus/lack of penalty when dealing with followers of other gods, as the Undivided champions are usually the ones who can unite multiple factiosn together.
Maybe we could come up with an undivided Rewards list, but one that you only gain access to at every other corruption threshold, rather than every one? So going aligned still grants easier access to better powers.