1: The entire point of Eevee is that it evolves into other Types. A Normal Evolution would be repetitive and fly in the face of that.
2: An Ability that gives Sylveon its own unique set of strengths and weaknesses would be... unique, but impractical. It would frankly be easier to just make it the only Fairy Type in the whole game. It would also be easier on players, since they could at least fit it snugly into a type-chart without having a unique section just for Sylveon.... A Sylveon-Type.
When? Because I've been seeing people claim they've said things on Smash that were never actually said.
The most concrete things that were said or shown on Smash:
- Sylveon is not a type currently represented by anything in the Eevee family, including Eevee. - A caption reading, "It's super effective!?" appeared when they tested Poison on Sylveon, implying that Poison is super effective on Sylveon's type.
Anything else could have just been for humor and theatrics.