I'm actually one of those under the impression that Sylveon will be a bug type; or some new type undiscovered yet. I watched another video (yes I know) but it didn't really give me many clues; just pointed things out. Take a good, hard, look at Sylveon.
Sylveon is the lightest eveelution yet; Sylveon has two bows appearing to be butterflies; Sylveon is a bright, peppy color; Sylveon's eyes are big and bright;
That's three marks. So you saw a video where there was a flying island or whatever with a rainbow. Are you sure it wasn't some pokefan wanting a flying type eveelution? I myself was gearing for a flying type, but with this concrete evidence I can't seem to steer myself. I'm pretty sure Sylveon is going to be a bug type. The name is French AND Japanese derive from Nymph; a small forest pixie like thing. Hence; bug type. But we will see.
Perhaps they'll create another Eveelution; they always seem to produce pairs; and then it'll be clearer. (The pairs typically contradict each other. Psychic - Dark; Grass - Ice. We'll probably get a hold of some bug and flying this time!
3 marks compared to flying's 259 marks!
Now don't exaggerate like that. There are certain traits in Sylveon that make me think it's probably a Flying type. But, Game Freak could pull a fast one and make it some other type just for the lulz.