The first and most important thing that you need to do before jumping into Mega Dimensions is make sure you have a solid team ...
A ton of Pokémon get new Mega Evolutions, like Mega Raichu X and Y. The Megas that were missing in the base game have also re-appeared (Mega Sceptile, Mega Blaziken, Mega Swampert, Mega Latios, Mega ...
A's first major DLC doesn't bring enough new ideas to the table, ultimately making it an unexciting remix of the base game.