Vaporeon with fins and Water Absorb, Flareon and its firey fur with Flash Fire, Jolteon and its spiky lightning with Volt Absorb, Leafeon with leafy ears and tail, Umbreon is jet black with red eyes, Espeon has a gem to imply supernaturalness and splitting tail for abnormality, Glaceon with (only) Snow Cloak) I feel that Eeveelutions are in a way the definition of the types they each represent, so while having wings doesn't defining a Flying Type, I really feel the Flying Eevee should have some sort of wing feature.
I'd say Types are either based on:
1) 'elemental' affiliations and other forms of power (such as Water, Fire, Electricity, Poison, Psychic, maybe Ground/Rock;)
2) zoological classifications (notably Bug, Flying, Dragon... I would also place Grass here because of their plant relation;) or
3) 'application' of their 'character' (which would explain for Fighting and Dark.)
The anomalies according to the above three points would probably be Steel and possibly Rock in a sense.
'Fairy' could probably go into either (2) or (3) IF this really is true lol (which I think Sylveon could be just to relate this post back to topic.) 'Light' is definitely (3).
And finally, with reason number (3), Game Freaks can create any Types they want haha, call it Space call it Astral, heck call it Smiling Type if you want! *Smiling attack is super effective on Houndoom!* lol
Sylveon can definitely own Umbreon.
Just some random ideas I cooked up to give Sylveon's possible new type a reasoning.