Assuming that fate and incestuous TV have already assured the middle child from Outnumbered, who has Just played William for Adam, and that Karen-from-Outnumbered's satanic abilities should be reserved for a 24-part adaptation of Paradise Lost, then I bring Youngest Sister and my longstanding fantasy casting to the lead roles:
Aziraphale - David Thewlis
Crowley - Paul Bettany
Just watch Gangster No. 1. They are fabulous together.