I've been working on this book for a while, but it's still a long way to completion. I'm not worried about spoilers, because it's still a long way to go.
In brief: I want to base my story around a 'main' central twist that recontextualizes everything, and sets the course for later books. But it's quite a major leap, so I'm wondering if it'll be a problem for readers.
The concept of the story is - The world of Endoria has a slumbering horror called The Wrack. Centuries ago, the goddess Asteria created the Radiant Seal that put it to sleep, the Kingdom of Illum (the continent's dominant power) are Asteria-worshippers and draw on the Radiant Seal to fuel their magics.
But the Radiant Seal is weakening, and the Wrack will break free within a year. The Temple of Asteria summons a Chosen One from another world to be their champion, and they get Gabriel, who is seemingly the perfect candidate. He's a college boy from the wrong side of the tracks, he's got a little brother who is dying of cancer and whom he wants to cure (because the slayer of the Wrack will be granted one wish).
Better yet, the Blessing of Asteria means Gabriel swiftly becomes an OP Archmage. Being a generally nice, contemplative and compassionate guy, Gabriel is soon hailed as the kingdom's savior and also ends up in a love triangle between the beautiful female knight he fights alongside and the Saintess who summoned him.
So after a volume of that, we see that another Chosen One - Darren - has been summoned on the far side of the kingdom, and he's an OP warrior blessed by Hecatoncheires, the God of Vengeance.
The big twist is that: Actually, Darren's sister (and Gabriel's ex-girlfriend) was in fact the Hero. Gabriel murdered her to steal her blessing, meaning he was summoned to Endoria in her place - Because he wanted to win the wish and save his little brother.
Darren, however, caught him in the act: In the struggle, the blessing of might and magic was torn in half, meaning that Gabriel has the 'magic' part and Darren has the 'might' part.
And now Darren is hunting Gabriel, seeking to win the wish for himself in order to resurrect his sister...But failing that, he wants to kill Gabriel and ruin everyone and everything he loves, and is raising an army of heretics, rebels and the dispossessed to do it.
Basically, this is a PvP isekai: Gabriel initially doesn't know Darren is in this world, too, but once he does suddenly the focus of the conflict becomes the two of them trying to destroy each other...while simultaneously fighting the forces of the Wrack, which is gradually becoming stronger.
Now, obviously, having the main character (and ostensible protagonist) being revealed to be a murderer - even a sympathetic murderer - is a big ask, especially when it means half the cast is directly invested in killing the other half.
Note that while Darren is in the 'moral' right, he's bitter, hateful, full of spite towards Gabriel, and doesn't care about Endoria other than a place he needs to get through in order to defeat the Wrack and resurrect his sister. Collateral damage is entirely fine to him.
Meanwhile, Gabriel is generally a good guy...Except for that one, terrible act he can never really make up for. So it's a modern "This is my new life" protagonist versus an old-school "I want to go home" protagonist, in a sense.
What do you think? Would readers go for it, or is this definitely going to lose them?
tl;dr - An OP Archmage story becomes a PvP story between an OP Archmage...and an OP Warrior.