can Mega Evolve!
They announced it pretty early on in Promotion of X & Y. On 11th February 2013, we first saw it in a magazine leak, with the 14th when we got the first official confirmation of it: https://www.serebii.net/archive/February-2013.shtml This was before we knew of the Fairy typingI agree. Slyveon was hinted about a month before XY were announced by the one promo sheet showing off new Pokémon merchandise having it blackened out and placed close to Eevee.
I personally doubt they are going to add a new Eeveelution just to be obtainable in the DLC.