Good!Light is a concept that you would need to develop yourself, since he doesn't really exist in canon, not in the sense that would try and make himself a true successor to L, anyway.
But you know that already.
So the AU aspect would be something that occurs before the warehouse confrontation to make him turn away from his life as Kira in favour of L's idea of what justice should be.
And then he'd need some way of proving that he wasn't Kira, in order to avoid having that same justice applied to him by Near/Mello et al.
But then, you know that too, so, I've sort of lost track of why I started writing this. Oh well...
SPOILIZERZ!!!!!!!!!
Good!Light is a concept that you would need to develop yourself, since he doesn't really exist in canon, not in the sense that would try and make himself a true successor to L, anyway.
But you know that already.
So the AU aspect would be something that occurs before the warehouse confrontation to make him turn away from his life as Kira in favour of L's idea of what justice should be.
And then he'd need some way of proving that he wasn't Kira, in order to avoid having that same justice applied to him by Near/Mello et al.
But then, you know that too, so, I've sort of lost track of why I started writing this. Oh well...