Changes Since Ruby/Sapphire

Mirage Spots: These are sky-based locations accessible by Soaring on Latias or Latios. They allow you to catch legendary Pokémon, potentially completing your legendary collection (excluding event legendaries).

Soaring: A new travel method where you ride Latias or Latios. It functions similarly to the Bicycle in Pokémon X/Y, allowing access to Mirage Spots and circumventing the grid-based movement system.

Horde Trainer Battles: An expansion of the Horde Battle concept from Pokémon X/Y, where you battle against five Trainer-owned Pokémon simultaneously. Strategies often involve widespreading moves.

New Pokémon: The National Pokédex now includes 721 Pokémon, with 719 available at the release of Pokémon Omega Ruby and Alpha Sapphire, an increase of 335 (87%) from the original 386.

New Formes: Similar to Deoxys and Pokémon from Diamond/Pearl onwards, some Pokémon now have multiple Formes that alter their abilities and stats. This includes superficial changes and more critical ones like Rotom's Formes.

New Type: Fairy: Introduced in Pokémon X/Y, the Fairy type is weak to Poison and Steel, and strong against Dark and Dragon. New Fairy Pokémon include Florges, Sylveon, and Xerneas, while existing Pokémon like Wigglytuff, Gardevoir, and Azumarill also gained this type.

Steel is Nerfed: The Steel type lost its resistances to Dark and Ghost in Pokémon X/Y, affecting Pokémon like Metagross, which is now weak to these types.

New Abilities: Hidden Abilities, introduced in Pokémon Black/White, can often be obtained through breeding techniques and offer significant benefits, such as Speed Boost on Blaziken.

New Items: Numerous new items have been added since Pokémon Ruby and Sapphire, particularly those from Diamond/Pearl that are relevant to competitive play, like Choice items and Focus Sashes.

TMs Have Changed: TMs can now be used an infinite number of times, and there are 100 TMs available, an increase from the previous 50.

Mega Evolution: Introduced in Pokémon X/Y, Mega Evolution allows a Pokémon to temporarily evolve mid-battle when holding a species-specific item and the player has the Mega Bracelet. This grants stat boosts and can alter types or abilities. Nearly 50 Mega Evolutions are available.

Primal Reversions: A special form of Mega Evolution exclusive to two specific Pokémon in Pokémon Omega Ruby and Alpha Sapphire. Primal Reversions are separate from Mega Evolutions, allowing a team to have both a Mega and a Primal Pokémon.

New Battle Types: In addition to Single, Double, and Multi Battles from Pokémon Ruby/Sapphire, Pokémon Black/White introduced Triple Battles (three Pokémon per side) and Rotation Battles.