Fun Fact: All Legendaries in ORAS are catchable before you beat the Elite 4 for the first time. So nothing new there.
Beating the Elite 4 for the second time will let you choose one of the Unova starters — Snivy, Tepig or Oshawott. Beating it for the third time will let you choose one of the Sinnoh starters — Turtwig, Chimchar or Piplup.
As far as my knowledge and experience goes, that's all Pokémon available after subsequent Elite 4 rounds. Hope this helps!