Well I actually joined this site due to this discussion
I may have overlooked it in this thread, but back when Sylveon was first announced I assumed dragon for a number of reasons.
1) The eyes (to me) look like dragon scales http://bulbapedia.bulbagarden.net/wiki/File:Bag_Dragon_Scale_Sprite.png
2) Though Milotic is a water type, it does have some dragon like attributes. Sylveon seems to share a similar color scheme.
3) The bows that are often viewed as being "butterfly-like" could also be seen as two scales back to back.
4) The color scheme for dragon-type attacks tend to be blue (royal and cyan), pink, and purple. (Sylveon shares many of these colors)
5) The wheel chart (the one that pairs Jolteon w/ Sylveon) assumes that it is suggesting a super-effective connection, but it could equally be resistance (or in my personal opinion just conjecture - the top 3 are generation 1, the middle 2 are generation 2, the last row before Sylveon is generation 4, and they all surround Eevee.) - Fire resists Ice, Dark is Immune to Psychic, Grass resists Water, which would leave Dragon, Grass, or Ground resisting Electric. All of the current Eeveelutions were Special types in the first generations - so that would rule out ground, grass = Leafeon, so we are left with Dragon.
Well that was what I thought then, now I'm a little less sure.
I'm not really comfortable with the talk about a pure female evolution requirement. As stated by a user above, the chance of receiving a female Eevee is a statistical improbability and in the case of the hidden ability gift Eevee in B2W2 it's impossible. If you've ever tried breeding for a female Eevee then you would know how frustrating it can be.
Aside from the statistics, Eevee just doesn't fit well into that female only category. The only Pokemon that seem to be female only, are female due to some reproduction aesthetic: Mammary glands( Miltank, Nidoqueen), child birthing capabilities (chansey, kangaskhan), etc. or by fitting into a female social role category (Lilligant = Princess, vespiquen = Queen). Sylveon doesn't fit into that category (at least based purely on aesthetics). It could very likely be just another cute Pokemon.
Many fans consider Espeon, Glaceon, etc. to be female because of their conditioned social role stereotypes, but the designers didn't agree that they were female only. Gardevoir's Japanese name is literally saanaito (Sir Knight) many declare Gardevoir as being female because of stereotypes.
Audino, Bellossom, Milotic, Luvdisc, Gorebyss, etc. etc. are not female only and one could argue that they are every bit as "feminine" as Sylveon.
In my honest opinion, I just can't see Sylveon fitting into any of the current types. I don't think a Light type is that great of a probability since Dark has nothing to do with the lack of light. In Japanese Dark = Aku which could be translated as Evil or Wickedness type. Also, Espeon is the Sun Pokemon and Umbreon is the Moon Pokemon - they already complete the light/darkness - good/evil dichotomy. In my humble opinion, adding a light type would feel rather redundant.
Lastly, could someone please explain the similarities between Skyla and Sylveon? Aside from the big eyes and a similar shade of blue, I simply cannot see it.
http://bulbapedia.bulbagarden.net/wiki/File:Black_White_Skyla.png