Honestly my ideal casting for Good Omens is to just… cast like 100 people each for Crowley and Aziraphale. So Aziraphale enters a room as Michael Sheen and leaves it as Richard Ayoade. Crowley switches from David Tennant to Riz Ahmed to Sue Perkins between cuts. This is literally never remarked upon by anyone. All the actors get a roughly equal amount of screentime.