It would be kind of weird to add them in some other expansion where both are not as relevant. Not that I would be against it, I just find it unlikely.
I wouldn't bother to wait if they would add the classes in the right way. Like giving more personal and characteristic animations, spell names and icons fitting the "faithful of the Loa" theme for Zandalari Paladins and a more "Dark/Twilight" vibe for Dark Iron Shamans.