Pokémon X and Y gave Pokémon battle mechanics quite a shake up three years ago when Game Freak introduced a new form of evolution, the Mega Evolution. This new mechanic allowed trainers to buff their best Pokémon by temporarily changing them into a super-powered form.
Mega Evolutions were a little incongruous with one of the main themes of the Pokémon series, which has always focused on long term grooming and training of your creatures in anticipation of the payoff of increased power and effectiveness. Some saw it as a cheap shortcut that didn't really fit in with Pokémon philosophy.
Fans of the Mega Evolution don't need to fear, the mechanic hasn't been scrapped, and Pokémon Sun and Moon will support Mega Evolutions for any Pokémon transferred into the game. But Mega-Evolution will no longer be a focus, so no new Mega-Evolutions are expected to be introduced.
Z latest thing
The focus of Pokémon Sun and Moon will instead be on the Z-moves. Z-moves combine the power of both the Pokémon and the trainer to unleash a powerful attack, but only once per battle.
Pokémon can only use a Z-Move if its Trainer is wearing a Z-Ring with a Z-Crystal set in it that matches the Z-Crystal the Pokémon holds. Pokémon director Junichi Masuda explained the change in focus:
There were Pokémon that weren't perhaps used too much in battle and they're perhaps going to come to the fore a bit more.
Pokémon that aren't normally so useful in battle can now have an edge with their powerful Z-moves, provided you give them the care and attention to collect the right items to match their abilities. The canonical justification for the introduction of Z-moves is also more in line with the spirit of the Pokémon story, focusing on an emotional bond between trainer and Pokémon. According to the developers:
The Z-moves themselves are also visually impressive, filling up the battle screen with their effects.