Putting aside the lack of red hair/green eyes, let's say Guy is Ingobert and Eugenie's son. That puts a whole new slant on the shitty way the Cecilles were treated, and gives a whole new angle/motivation to Guy's revenge plans.
Guy/Nat could still happen because they still wouldn't be biologically related, and it could definitely have a darker tone to it. But the whole thing would be kinda Alistair/Anora, wouldn't it? The bastard son who doesn't want to be king, and the girl with no blood claim to the throne who's actually a good ruler and beloved by the people.
Dang, that could be the best AU fic I'd never write.