Update (Feb 14th): Ninfia’s English name is Sylveon. More info available here.
The full name of the next upcoming Pokémon movie is “Extreme Speed Genesect and the Awakening of Mewtwo”, according to the latest issue of Japan’s CoroCoro magazine. Prior to the main film, a short titled “Eevee & Friends” will be shown, which will also feature a new Pokémon.
The above is the cover from CoroCoro, which contains information on the new Pokémon. The black silhouette you see on the cover is of the new Pokémon that will be in the movie.
Meanwhile, this image depicts the new Pokémon in full, with the Japanese name, Ninfia. Ninfia is reportedly an evolution of Eevee. Its type has not yet been revealed: