Nintendo have now confirmed details for Mega Metagross, which was recently leaked via CoroCoro.
Upon Mega-Evolving, it gains the ability Tough Claws and remains Steel/Psychic type. Its Speed is greatly increased, but also has small increases in each of its other stats too. More pictures and video below.
Also confirmed are some gym leaders and Elite Four members. These are the same as in the original games, but we see that Elite Four Phoebe uses a Dusknoir, Elite Four Sidney uses a Shiftry, and Champion Steven uses Mega Metagross.