To be honest, I have mixed feelings about Sylveon. Don't get me wrong, it's awesome that we get another Eeveelution but it's overall design is a bit too... girlish to me.
Still I'm eager to see what kind of stat's it'll have in the end and try it out.
Also I'm hoping that we'd get another Eeveelution. It's not just because "they have been released in pairs earlier" but because I've always wanted to have Dragon/Poison/Steel-typed Eeveelution. Also, as Fairy is quite a powerful it would be a great way to kind of balance the whole Eeveelution chart a bit atleast.
I know that only Sylveon was shown in that Pikachu Short Movie but if you think about it, it won't be impossible to get another Eeveelution. We was shown for example only Mewtwo Y in the new movie and still we got Mewtwo X too so it could happen. Anyhow, I'm still satisfied if we only get Sylveon - though I hope I can find a shiny Eevee so I get Sylveon as blue coloured. :D