There's apparently rumors that Famitsu will be revealing a new Pokemon next week or something...which could be the Pokemon Smash Announcement this week.
Since the Pokemon Smash Preview showed a bunch of movie Pokemon, is it possible that Movie 15 might feature a brand-new Pokemon from Gen VI? Even though Gen IV waited till Movie #4 (13) to show off the first Gen V Pokemon, Zorua and Zoroark, Gen II and Gen III featured new Pokemon (Kecleon, Azurill, and Wailmer in Celebi's Mini-Movie, although Kecleon debuted earlier in the show and Munchlax in Deoxys), so it's not too early.
Well, if we bring out the Nerdy Fan Pattern Recognition, the first Gen III Pokémon were revealed for Gen II's second movie, the first Gen IV Pokémon was revealed for Gen III's third movie, and the first Gen V Pokémon was revealed for Gen IV's 4th movie.
Now, if we assume that Gen V was a blip, it would make a certain amount of sense for a Gen VI Pokémon to be revealed in the Keldeo movie, as it's the second movie of the generation. However, there is another pattern; Pokémon from the next generation tend only to be revealed when there are no more Pokémon from the current one left to go. Ignoring the reveals of Gen II Pokémon, as almightily screwed up as they were, and we're left with the last Gen II Pokémon being revealed at the same time as the first Gen III ones (Celebi and Azurill/Wynaut/Kecleon), and the last Gen III Pokémon being revealed at the same time as the first Gen IV one (Deoxys and Munchlax). Now, Gen V breaks the pattern a bit, only debuting a new Pokémon a year after the last Gen IV Pokémon was revealed (Arceus). However, the fourth gen movies had a bit of a different thing going on, with the conclusion of the arc plot running across the three movies, something there is no sign of with the Best Wishes movies. The Arceus movie was packed enough, without throwing extra Pokémon into places where, the logic of the series dictates, they should not be (Unova being a clean break from the other regions and all).
TLDR: I don't think we'll see a properly new Pokémon in the fifteenth movie, as Genesect is still yet to be revealed to fill up the Gen V roster. However, I will be very surprised indeed if we do not see a Gen VI Pokémon of some kind next year. In fact, I would not be surprised to see a Gen VI Pokémon headlining next summer, a la Lucario or Zoroark. Whereas the movies gave Arceus space on its own to highlight its importance as a Pokémon, Genesect really doesn't require that at all; in fact, I'd argue that its backstory would make a more suitable culmination for the plot that Team Rocket's been putting together in the anime; fossils, revival technology, massive amounts of electricity. I would be unsurprised if Genesect appeared with Plasma in the anime this winter, leaving space for a Gen VI blowout next year; the way they've hurriedly dealt with Keldeo, Kyurem's formes and Meloetta in one movie certainly suggests to me that they're hurrying things along.
In other news, HOLY SHIT MUNCHLAX WAS REVEALED NEARLY EIGHT YEARS AGO. 0_o