Mega Evolutions has been removed and replaced by the Dream/Nightmare Forms.
The player can access the Dream or Nightmare worlds during the Night.
However the trainer can't catch Pokémon or level-up Pokémon during those sequences, however the trainer can store "Dream/Nightmare Memories", which can be used to train the Pokémon later.
There's now a bigger connection with the trainer and the player, the player can interact with the Pokémon during battles, the player can also talk with the Pokémon via telepathy, creating a link between the trainer and the Pokémon, by giving the Dream/Nightmare experiences to the Pokémon, which can increase one of the Pokémon's stats, personality also affects during the Pokémon and trainer relationship, the relationship can be also affect battles, the bigger the relationship, the better the Pokémon will act during battles, the lower the relationship, the Pokémon will not respond to your actions for the most of the times.
Pokémon can become hungry, bored, happy, sad, mad, etc..., this will be shown via an icon on the Pokémon MS icon.