While it would have been fun if Rob Benedict had played God on Lucifer, I’m sure he’s relieved to be taking on a different role.
There’s also the fact that the role of God has already been filled by 24 vet Dennis Haysbert (sorry, Rob!). Haysbert’s iteration of the character is set to appear throughout Season 5.
I think it would be pretty fun if he and Benedict’s Vincent Le Mec just so happened to bump into each other in the second half of the season.