(TL;DR) Divinity Form would essentially be a transformation, a source costing, mid game or so power, that offers substantial power, and would be very thematic.
I thought I would suggest this, as the game isn't out yet, I'm unaware if a feature like this is in the game, or if it was considered and dismissed, etc.
Idea is simply put, give each character (custom or origin character) a new 'form' unlocked eventually throughout the game, it would be based either on a God, or on their origin story. Lohsee's might resemble a Demon (like she transforms into an archetypal Demon, say the size of the female Lizard), gains a new ability or two, requires source to use, gives free armor, magic armor, fire heals, something similar. Ifan's might be almost werewolf-like (he has a spirit wolf familiar so I'm assuming he has an affinity for wolves), but would be clearly more God-like than a Werewolf. The Red Prince might solidify a pact with a Demon, (unlike Lohsee who would essentially become the Demon), he'd grow taller, gain horns, maybe even grow wings (it'd look pretty cool).
Custom characters might gain new Divinity forms based on their God, with potential changes available based on player choices in the game (input a minimal karma system that measures you on a scale of good to evil, stealing is 1 point towards evil, murder is 5 points, healing someone that is otherwise going to die is 5 points towards good, etc.), so for example on the minimalist side, the karma side determines if you turn into an evil or good god, (of your race), with slight variation, perhaps a god of creation if you are good, and a god of destruction if you are evil, with suiting abilities.
Of course, such a change would be kind of concerning, how do we balance this around players? How much will it cost to implement, how long would it take to implement and balance, these are all of course very reasonable questions, that I don't have the answer to. :)
But, if nothing else you could simply tweak the stats of the enemies by like 10-15% (perhaps more for bosses) to make the overall content harder after obtaining the new form(s), wouldn't be perfect, but would definitely be an interesting change.
The origin characters should probably have the option to get the regular divinity form instead, if they at some point, were given the option to turn from their path (like if Lohsee purges the Demon from herself, the Red Prince rejects all demons, etc.).
A very nefarious idea crossed my mind, when the God(s) or w/ever give them this power, they should be warned with something like 'Using this power comes at a great cost, in the end you may regret it...'.
Now as most people are probably like me and will spam it, it would be interesting to have a powerful (presumably new) foe eventually cross paths just before the end of the game, whose power is based off of the number of transformations.
This should occur as a group based fight, where they eventually 'face themselves' on their last leg.
Perhaps each player would have to fight their own 'self' or something similar, a reverberation and a cost of using their power. The custom characters will fight the opposite alignment version of their 'Divinity' aka Creation vs Destruction, and the Origin characters would fight distinct versions based on their origin and alignment. Someone like Fayn could turn to good, and end up fighting his darker nature essentially. Lohsee may end up fighting the demon for control, or becoming the demon and fighting herself (essentially).
Something like this could even serve as the moment when they unlock the form, where they choose a side, and fight for it. The new monster might defeat the entire group in one hit, where they fall to their knees and face a choice, and then that choice unlocks the Divine Form. Red Prince faced with the choice might say 'I cavorted with Demons once before, and was cast out for it. Do not mistake me, I will reclaim my throne, but, I'm afraid I will not be making the same choices as before.' (Rejecting the offer of the Demon and accepting the power of his God) Alternatively, he might choose 'You offer me the power needed to reclaim my throne? So be it, I accept.' (Accepting the offer of the Demon and rejecting the offer of the Divinity) Similar situations could be made for each character, obviously I don't know how this would impact the story and whatnot, but I think it'd be an interesting idea to consider. If nothing else, it could be put in after the main game is finished (as DLC or added content), or put into the next game.
