Every single Gym Leader, Rival, and trainer has been manually redesigned. They will use 6-Pokémon teams earlier than usual, proper competitive held items, and actual synergy strategies.
: The only Pokémon missing from wild grass are Kanto/Kalos starters and legendaries that were not originally part of the Omega Ruby/Alpha Sapphire (ORAS) base game.
Many trade-only evolutions are replaced by level-ups or item use (e.g., Haunter evolves at Level 36 at night).