The Guardians of Donna Noble

This takes place after Armageddon.
Aziraphale is made a guardian angel, he is assigned to guard a human being called Donna Noble.
Soon after Aziraphale is made a guardian angel he tells Crowley about it and Crowley ends up following Aziraphale to guard Donna Noble and helps Aziraphale guard Donna, hence, Crowley becomes a guardian of Donna along with Aziraphale.

In my AU: Crowley and Aziraphale are married but heaven and hell knew they had no chance with stopping it so they allowed Aziraphale and Crowley to get married.