I don't think there'll be new evolutions of anything unless they have an evolution theme going on. Which is actually quite likely, since X and Y point to genetics...
Evolutions are a way to balance bad Pokémon. In older gens this was necessary, for example in 1st gen, Pokémon like Seadra and Onix were quite bad, so they needed evolutions. Prior to 4th gen there were a lot of Pokémon like Tangela and Lickitung which were fairly useless, and got new evos.
But 4th gen was an interesting one because the whole theme of the series was evolution (Professor Rowan being an evolution specialist) and there were more than 10 new evolutions introduced. Technically, anything with a BST of 500 or less which hasn't yet evolved twice can get new evos.
As for Eevee, they need a new method of evolution for a new stage. Maybe a new item as well.