Some of Sylveon's foreign names contain "Nymph" or "Ninf" which comes from the creatures called nymphs in Greek mythology, who are basically fairies. I've also seen from other threads that "Sylph" is some fairy crap, too. A common portrayal of fairies is a ball if light. (Ex: Tinkerbell in Peter Pan)
All of previous types of the Eeveelutions were special before the D/P split. The only they could kind of keep this pattern with Sylveon being dragon, or creating a new type that probably would've been special. I think we all know that Sylveon probably WON'T be dragon.
They haven't revealed the type of Sylveon yet. There has to be a reason for it. It's either GameFreak is being TrollFreak, or the type itself is a secret. The only reason a type would be a secret is if it were new. It's not like they have to tell us "Hey, flying is in this game!"
There has also been the idea of a new type floating around for a while now... in fact, some thought Reshiram might have it...