I've thought that if there was going to be another Eeveelution in this generation, then it could be a male counterpart to Sylveon. Since it has a very feminine look, there could be another Eeveelution to pair up with it since most Eeveelutions have been introduced in pairs.
Evolutions have only ever been added twice, that's not really enough to say that they will always have 2 Eevee evolutions.
Eevee itself is really popular. If they sought that popularity to introduce a new Evolution, WHY just 1? Marketing wise it would have been better to show them both if there were two. So far the info we have says that Eevee only has 1 new Evolution. Coro Coro even has a picture that lists Sylveon as the only new evolution. By that I mean it as an arrow pointing to each evolution form and to Sylveon. Sylveon's arrow says new Evolution. There isn't any other silhouette on there or anything to refute that we only have 1 Evolution this time.
That being said, there are now 8 Evolutions of Eevee. 9 Forms of Eevee. 7 of Eevee's Evolutions are "Special" Typed. There is only 1 Special type left to be represented but as some have raised the issue, it doesn't really look like a Dragon.
Well, we have a lot of Pokemon that do not look like the type the represent. Jynx by no means represents Ice OR Psychic. There isn't much about her that screams Ice type nor does she hold that mystical and mysterious look that Psychics have.
Altaria doesn't look much like a Dragon type. Altaria is one of my more favorite Dragons, but she looks as if she should have been Normal Type rather than Dragon.
Stunfisk looks nothing like a Ground nor an Electric type. Its a fish that can be found in special puddles in Gen 5, so technically it lives in water. But it is not a Water type. But its a fish that lives in water.
Hoppip and its evolutionary family are all Grass/Flying types. None of which look as if they belong in with the Flying types. They lack all necessary appendages and do not have any real lore to back up the tag on.
Trapinch and Vibrava are both extremely insectoid looking Pokemon of Ground and Ground/Dragon persuasion. Neither look as if they should be a ground type and Vibrava definitely does not have a dragonesque appearance.
Slowpoke and Slowbro are both psychic types. Their Pokedex data state how dumb and slow both are which seems to contradict the information most other psychic have as all other psychics are extremely intelligent. Even Psyduck has a better psychic back ground than Slowpoke and Slowbro.
The only other one I really want to touch up on is another appearance one.
People believe Sylveon may be Flying type because of the "kite tail" relationship between her and her ribbons. Ok. Only thing I have to add to that is Shiftry. Sylveon has been rumored to be related to Sylphs, Wind Spirits. Shiftry is based off of a Tengu. Tengu are a special classed Japanese demon that is usually worshiped as Gods by some regions. They are normally "Bird Demons" and have power over "Wind" through use of their own magic or through a magical fan like item.
Shiftry's level attack list usually has something to do with Storms or Wind. Like Sylveon it has an origin based on a wind creature. Its type is Grass/Dark. This means that Origin doesn't always associate with type. Sylveon can still be a Wind oriented Pokemon of Normal typing as a few Wind based moves are Normal type.
Other than Normal and Flying, the only other type that has outside relation to wind is Dragon through various lores. Most Chinese Dragons represent one of two basic elements, Water and Wind. Many Dragons are bringer of storms and are wind based. Western Dragons who are more commonly associated with Fire also have some attributes of Wind. Japanese Dragons cover all known Magical and natural elements so there can be anything Dragon related so long as they want it to be.
The three I keep thinking are Normal with highest chance that I can foresee. Dragon being second because of something others pointed out due to color. Third is Flying, which I can only see if they tag it on as a secondary type. Everything else doesn't seem to have enough evidence to convince me of it being possible.