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What you all need to remember with eeveelutions is that they all have some kind of representation or embodiment of their typing in their design. This is usually their most prominent feature (except for maybe Espeon, whose feature would be the small gem on it's forehead)
Now, let's take a look at each of the popular speculated types for Sylveon
>Flying
This one is the one I think is most likely. While some people would argue that it couldn't be this because it doesn't have wings or anything that could let it fly, you have to remember that Flying is not just a representation of flight, but also the air or wind. But how do you represent or embody an invisible force? Simple, you show its effect. Sylveon's ribbons are one of the things that really stand out, and they are constantly shown looking like they're fluttering in the breeze. Combine that its light weight and Flying seems very plausible.
>Bug
This one I find less likely, but still possible. The "butterfly bows" could be a representation of the type, but it's color doesn't seem to fit, and I would expect it to have more insect-esque features.
>Dragon
Alright, now we're starting to grasp at straws here. Looking over Sylveon, I can't find any embodiment or representation of dragons or mythological creatures in general. Not very likely.
>New Type
With a new type, anyone can just say, "This is the new type, and pokemon of the type have these features. Oh look, Sylveon has those features, it must be my new type!". Since Sylveon doesn't seem to just click into any type niches for people, they end up thinking that they'll be a new type that it will work perfectly with. All this being further fueled by the emphasis they put on the mystery of its typing in CoroCoro, although that could just have been a strategy to keep us discussing what it could be, keeping the hype up.
>tl;dr no one knows what Sylveon's type is still, but here are my opinions
