Rock types are more rough around the edges, which Sylveon is definitely not. I think Rock and Ground can easily be ruled out here. Along with every type that has already been taken by the other Eeveelutions. I'm also ruling out Normal, because I doubt that if Eevee were to ever have a Normal-type evolution, it wouldn't look so different. Also, from the latest trailer, it seems to imply that Sylveon is anything but normal.