This can be a crossover with any Isekai story, but what if Ichigo, after the Winter War, when he’s alone and without most of his soul, falls into a different world with magic and monsters?
He’s still very depressed at first because he has no soul, no powers. He’s used to no friends or family, but then he opens his inventory (it takes him a while to figure out how because this thing doesn't come with a manual), and he has two level-locked items.
He can't see them, but he’s certain they're Zangetsu and the Hollow, which he is surprisingly happy to have back. He plunges himself from one battle to another in a constant desperate bid to gain more levels. He doesn't care how injured he gets. He *needs* Zangetsu and the Hollow.
He fights opponents massively above his level without hesitation or fear because he realises that will make him level up faster and needs his powers back. He needs his soul back. He doesn't stop for a moment to heal or rest; he just keeps fighting, challenging stronger and stronger opponents to get those level-locked items.