Man, the possibilities with that premise are endless, but one of the most unsettling ones I've seen hinges on the Soul King being less a cosmic entity and more a state of being, a role that must be filled. The twist is that the 'Soul King' Ichigo finds isn't some ancient, suffering prisoner, but a relatively recent Shinigami who failed in their duty, trapped there as a warning. So the throne isn't a source of power, it's the ultimate punishment.
Ichigo's whole goal shifts from 'save the Soul King' to 'stop anyone from making the same mistake and getting stuck here forever'. It flips the rescue mission into a desperate race to destroy the system itself before his friends try to 'save' him by putting him on the throne. It's a bleak inversion of every 'Ichigo becomes God' story, where ascension is the worst possible fate.
Seen one that played with Uryu's role beautifully. It wasn't about a big reveal of lineage, but a quiet, horrific understanding that the Soul King's existence is what maintains the separation between worlds, and Quincy power, which erases Hollows, is the only force that can truly 'cleanse' the cycle enough to make the throne obsolete. So the twist was that Yhwach was right in his goal, but monstrous in his method. Ichigo realizing that his Quincy friend might hold the key to a peaceful dismantling, but using it would mean undoing Uryu's entire identity as a survivor of genocide? That's a personal conflict you can really chew on.
For a shorter, punchier take, a story that recontextualized the Hollow within him as the original, failed Soul King's will, a fragmented consciousness left behind. It made every time Zangetsu took over not just a power-up, but a slow, subconscious reclamation. The twist was that becoming the Soul King wouldn't be Ichigo taking a throne, but the old King using Ichigo's body as a vessel to finally escape. You lose the protagonist in the most literal sense.