Casting HBO's Justice League Dark

The Jason Blood/Etrigan dynamic is not quite as disparate as the Bruce Banner/Hulk one, so the choice to have Michael Sheen and David Tennant respectively play the two sides of the character wouldn't exactly be for the purpose of having them be the opposites they were in Good Omens.

The association with those two characters would certainly help to illustrate the differences between Blood and Etrigan though.

Sheen would work as Blood as he would be able to convey the idea of him being a good guy, while also portraying how the character can also seem rather menacing (just look at him in Apostle for instance).

Going back to Good Omens, Tennant would simply be fun as Etrigan - he may not look the part, but then again, there aren't many actors that look like massive, yellow demons.