As most people know, Eevee and its evolutions have been heavily promoted in Japan, both in the anime but even more so in merchandise releases. Some fans think that this is nothing more than a nod to Eevee being available in B2W2, while others think that there doesn't have to be a reason for a marketable Pokémon to be promoted. But personally, I don't find either of those explanations satisfactory, since the current Eevee promotion far eclipses its promotion during Generation IV, where it was more relevant due to having two new evolutions and being featured in Platinum as well as HGSS. That is why I've been suspicious that a new Eeveelution may be in the cards, with the anime using it as a plot device for the Unova League climax.
Up until now, I've found no evidence to substantiate this theory. But that has now changed with the announcement of a lottery-based Pokémon Center promotion, which involves some very interesting postcards:
Those postcards will be released from December 8th to December 14th. What does the question mark represent? Eevee and the seven Eeveelutions are accounted for, and there is no reason why an unrelated Pokémon would be involved here. While December 8th leaves no time for a CoroCoro announcement, we do know that next week's anime episode will be followed by a TV announcement. Granted, as far as we know the information will be about Episode N and possibly Movie 16, but there is actually no reason why an Eeveelution couldn't be revealed at the same time if it were relevant to the climax of the Unova League.
Edit: Unfortunately, it turns out that the postcards were previously released last month, and the "secret" card merely contains an ensemble picture of Eevee and the seven Eeveelutions.