Sylveon's appearance doesn't look like it matches any of the current types. With the most recent Smash episode there are certainly some we can cross out though.
It's strong against Ice types, so it can either be Steel, Rock, Fighting, Or Fire. However, since we already have Flareon then we can cross Fire out of the list. And it can't be a Dragon, Flying or Ground type if it's strong against Ice, and Bugs aren't effective against Ice types either.
We know it can be affected by Poison types, so Steel is out. It's also weak against Fighting types so Fighting can also be ruled out as well as Ghost.
I find it hard to believe it will be a Rock type, so I feel like that can be scratched off the list too. Also, since it has a strength then that crosses off being a Normal type.
All that leaves is a new type, which I feel is a real possibility right now. There's also a dual type, but like being a Rock type, I find it hard to believe GameFreak will break that tradition.
PS. When I mentioned not discussing the name of the type, that's exactly what I meant. The name. If you're only talking about what the name of the new type can be and not how it relates to Sylveon, then that can fit in the thread about a new type rather than go here. Otherwise, things may really venture off topic. It was just a friendly reminder.