I hope we won't see belief = power, myself. Torment and Mask of the Betrayer both covered it from two different angles anyway, I'd rather see something new. I expect, given the way souls work, that we'll see something that links gods to the cycle of souls. For example, does the cycle just happen or are there psychopomps who carry souls into and out of the world? (Physical gods seem unlikely to me since I think we heard in a prior update that dead souls went to wherever gods are). Maybe there's a warrior goddess or something that guards the door back into life to make sure that a soul, and not something else, gets to inhabit a new body.
The general wonkiness that can happen to souls (fragmenting, co-habitation, etc) makes me think that apotheosis is likely possible, but maybe in more of a buddhist way, where it requires lots of different lifetimes lived. I also like the idea that a few people have had that gods are large accumulations of souls in some way. One take would be two classes of gods, one representing natural processes (Death, weather, fate, the ocean, etc) and another class representing human concepts (war, love, wisdom, luck, etc), the latter group forming gradually to provide a safe harbor for human souls to rest in.
One thing I really hope Obsidian does is find a good way to work "institutions of faith" into the setting, though since souls are such a huge deal in the setting they've got to be thinking about it. The catholic church tends to get transplanted a lot, but that's getting a little old. The setting seems to lend itself to Tulkus, for example. The cycle of souls is a good opportunity for significant birth and death rites. Maybe there's a superstition to help your kid attract the kind of soul you want, or repel the messed up ones, and maybe people about to die are given messages to take to the gods, and safeguard the state of their soul.
Finally, sin-eaters are cool, and while not directly suggested by the soul thing, could have interesting consequences for the whole cycle, and I've never seen them in a setting before.