It is still a possibility though. Didn't they not reveal Glaceon and Leafeon before the Gen V release? They could be intentionally keeping the Dragon version in the dark since the introduction of the Fairy type is the biggest change and the one they want us to talk about. Giving us a cool surprise as well.
Not saying it's probable, just possible. They may never since they probably don't feel any obligation to give Eevee an evolution for all or most types, and the method to evolve Eevee into a Dragon type would probably involve an older method (level up in some draconian place) instead of a new method like the whole bonding thing that likely is the route to Sylveon.
We can't say for sure Sylveon is the only new evolution of Eevee but we also can't really say we should expect another one either.
Faust_8: Would you own slaves if you could? OW: If I lived on a farm or something, sure. But I have no need of slaves right now.
I suppose it's possible, but it seems counter intuitive to develop a Dragon form for Eevee when your big promotional boom is introducing a type meant to combat Dragons. I'd be more interested in seeing something a little less mundane, like a Flying or Bug type (seriously, another pure Flying type without the slap in the face of Normal subbing).
I remember a rumour before Fairy type was revealed that X and Y would be themed on Fairy and Dragon. Thus, we have seen the Dragon symbol on TCG cards. Yes, Eevee would eventually get a dragon evolution (which better be good)
Gamefreak better add a feature that can topple this fairy type.