Metallo can work. And I think he will be used.
Don't expect someone like Brainiac already at this early stage. The exploration of Superman villains in films have gone extremely slow. SR used only Lex, not much of evolution even after almost 20 years of absence. And MOS did not exactly come up with something new in terms of picking a villain, they re-used Zod (although a more developped version).
To sum it up, it will be a brave move to even have someone as "basic" as Metallo in WF. He's just a metal robot with a kryptonite core.
Brainiac (a genius A.I. being), Doomsday (the most deadly creature in the universe) or Darkseid (a "god")... sorry, it just won't happen at this stage.
EDIT: Wait, didn't they use Brainiac in S III, sort of?