The only totally new areas available in the post-game of ORAS are the Battle Resort and Sky Pillar. You can access the Battle Resort after the Delta Episode, when Norman gives you the SS Ticket. Sky Pillar is accessed at the end of the Delta Episode. Technically you can also go to to outer space after catching Rayquaza, but only once.
EDIT: While accessible before the Pokémon League, if it's Mirage Spots you're looking for, look no further.