A hacker by the name of Smealum appears to have discovered some new Pokemon and Mega Evolutions including three event legendary Pokemon: Diancie, Volcanion, and Hoopa.
In an effort to keep this all a surprise for those who wish to avoid spoilers, you'll need to visit the gallery or links below to see this stuff. This information is unconfirmed, of course, but it does look to be genuine. I'm so preoccupied with the existing Pokedex that I don't even want to think about more legendaries at this point.
There's also two more wing patterns for Vivillon. Because they're aren't enough of those already. (Truthfully, the collector in me loves these silly butterflies.)