Not really, since all of the Eeveelutions have involved new types of evolution. Gen I: Stones; Gen II: friendship and day/night cycle; Gen IV: Location based evolution.
I'd assume there'd be a new evolution method that effects Sylveon's evolution; although, there's unused styles for Eevee (trade (w/item), learned move, held item)
It's weird-er but still not overly strange for all of the Gen IV evolutions that popped up for Gen I Pokemon. Although, a lot of them were learned move (new for that gen, IIRC) or used new items to make the Pokemon evolve.
But I suppose they could just do this for Mewtwo.