I think part of the issue is that the most powerful (mortal) person in the entire game as it currently exists is Petrus so we don't really have anything to aim higher for except the gods.
Ischaldirh wrote
I suppose I could see an ending where you become the avatar of a deity - effectively becoming their impersonation on the physical realm... Or perhaps gaining divine immortality and some sort of demi-god status.
I think the current "best" endings currently fill this role, or come close, since they involve becoming the champion of either Mortifer or Valpurus.
I'm personally thinking the next logical step up the ladder is to challenge them, or the other gods.
Maybe something less silly than fighting a god would be to become the champion of X god, and then beat down the champion of Y god on the other end of the good/evil scale?
Ischaldirh wrote
But actually facing - and slaying - an immortal, divine being? I dunno. I just don't see it.
That's what makes it an ultra-ending. It's near impossible and ridiculous.
Facing a god is easy enough, especially given the whole physical presence thing for a few of them. As it is, Valpurus can severely damage you by doing even less than shrugging his shoulders.
Beating them in combat is an entirely different story, and should be so insanely unlikely that doing so
deserves a unique ending.
We also don't know that they're immortal, because the game lore doesn't elaborate. Hell, we don't even know how the gods work other than the fact that they can conjure up (or teleport in) items, limbs and NPCs out of nowhere when you ask them to.
For all we know Infuscor is an insanely powerful dark mage who locked herself in some arcane library and communicates via world-spanning ESP.
As a side note, I'm not trying to be instantly dismissing of your ideas because I do think they have merit. I just think that having a ridiculously over the top scenario for the best possible score multiplier ending in the game would be super cool.
I also like the idea of, say, any character being able to approach a God and instantly being fried for so much as looking at them the wrong way without proper preparation.