We know from LoK that a previously severed bending can be restored, right? We only see this being done by energybending masters, but what if finding a new source for your element, and thus effectively creating an entirely new bond with your energy, could somehow do the trick too?
Basically I want Iroh to take Ozai on a long and torturous journey of repentance and self-discovery and mutual forgiveness that ends back at the Sun Temple, where Ozai has to face Ran and Shaw's judgement. And like for a moment he's sure he's going to be burnt to a crisp, maybe he even thinks this was all a weirdly elaborate setup to get rid of him for good, but by then he's already changed so much, and his brother is by his side, and he's learnt to rely on him and trust him all over again, and after all the dragons already bestowed their ancient knowledge on an arrogant and power-hungry fire royal once, in another lifetime. Maybe Iroh drags Ozai into doing The Dancing Dragon with him. And at the end of the trial, when they're both engulfed by the multicoloured flames, Ozai's fire awakens.