Pokémon Omega Ruby and Alpha Sapphire/Changes from Ruby and Sapphire

12 years from 2014 went out Pokémon Ruby and Sapphire, and since those years, many things have changed in the main core... So, even if the Remake is quite faithful, many new features come extend the game. Let's give a look to the most important points! For a complete list, you can look

Mega Evolution
This feature, introduced in XY, is used in ORAS. New Mega Evolutions were introduced, they are presented in Mega Evolutions. To make a Pokemon Mega evolve, you have to find his stone and active it during the battle. The mega evolution is a temporary changing, which means that, at the end of the battle, your Pokémon will find his normal form back.

A new type
The Fairy type is new since XY, so it wasn't in the original games, but incorporated in the remakes. Some old Pokémon take this type, often instead of normal type. This type is strong against Dragon, Fighting and Dark type,and weak to Poison and Steel. This means that typetable is not the same in the two games.

Five against one
Occasionnaly, you'll have to battle a horde of five weak Pokémons, with only one. It's a good way to gain a lot of experience in few time.

A longer Hoenn Pokédex
We discovered new evolutions for some Pokémon of the Hoenn Pokédex in the IV generation. They are now integreted in the Hoenn Pokédex, which is therefore longer. The Moss Rock to make evolve into  can be found in the Petalburg Woods. The Ice Rock for is to be found in Shoal Cave. To evolve and, you have to go in New Mauville.

HMs, TMs, and moves
Since Generation V, TMs can be used indefinitly. Flash is no longer a HM in the remake, so it doesn't require the Knuckle Badge to be used. The distinction between physical and special moves are no longer made following the type of the attack, but is particular to each move. New moves introduced since Generation IV are, of course, present in the game. To have a complete list, give a look in the List of Moves.

Time is flowing
The Generation III games didn't use day/night system introduced in Generation II. But since Generation IV game, this features is always present.