I'm not a particularly skilled writer but it feels like there is space for a genuine corrupt route, in particular a "palpatine" plot. The cult leaders goal as far as I'm currently aware is basically he's going to drain kas power into himself, not counting any possible backstabs etc along the way. You can make friends and political alliances the entire game, and while of course theyd probably throw deus ex machinas in to prevent it or semi reasonable explanations like gods detecting something wrong, there seems to be a opening for the mc to be a heroic paragon of justice and love while taking the cults research, preventing or altering the attempt to take kas' power, and taking it for yourself.
Which is very very important as a plotline, because in one of the bad ends (minus their "not canon" comments) is a ending where kas has essentially absorbed all the gods and probably the "thing in between"'s power and is essentially hollow from it, presumably from the time gap between seeing your character, who was frozen, which could be thousands of years etc depending on game magic logic, and probably her gambit to have children being unable to work anymore for various reasons, maybe she got hp Lovecraft style knowledge that broke her or made her like a computer.
A lot of text but my point from that is imo there's a solid setup for a undertale-like genocide route, a totally optional, underlined "if you do this you will betray all your friends and those who've put faith in you", where you take kas' power and presumably nobody knows. You leave your friends, maybe you get a expanded power set like the batter from off, and you do a fast surprise attack, let's say the kitsune den first. You face the npcs, but more importantly you face kiyoko and kinu, characters that (possibly) you're more connected to, and they get justifiably hurt and angry. You slaughter/defeat kitsune, corrupt them until keros appears. You make keros a superboss, you absorb him, and you use that power to leapfrog to the next zone, each zone having minor npcs and npcs you've probably liked before, corrupting party members into the forms kas shown you in dreams, their personalities becoming distorted and broken, your power skyrocketing, encounters with power far beyond what you would normally encounter, until finally you sit on a throne, but at what cost? Maybe you have 100s of sloots, minorly altered husbands or wives, but no equals. You betrayed kas to fulfill your goals, your party has been altered to submit to your will, and maybe the entire world. A puppet master with only an eternal puppet play. *Babam* corruption of champions 2
