Keldeo is already revealed, people. That's actually pretty old.
The stand for the five mystery Pokémon was shown in some merchandise convention -- that was before the announcement of Black 2 and White 2 and the Meloetta short, but after the revelation of Keldeo. That means, that from then until now, three have been revealed:
Black Kyurem, White Kyurem and Meloetta Aria Forme.
Obviously, we can say safely that one of the still-mysteries is Meloetta Pirouette Forme. But that still leaves one open spot.
We've discussed some theories on what this could be with a friend. It should be first and foremost be said that there have been "mystery Pokémon" in the past which turned out to be just old Pokémon that had an appearance in the movie. Taking this into consideration, these are the other most likely possibilities:
* A Spikey-Ear Pichu-esque Pokémon; an old Pokémon with a distinguishing characterestic and/or a special role. Might as well be a shiny Pokémon, like Pikachu-colored Pichu (or even a normal Pokémon that will have a special role). In this case, we will probably also see a (probably) non-Legendary Pokémon event in the games as well. I personally like the fact that downloadable events aren't limited to Legendary Pokémon anymore.
* Genesect; unlikely. The film is already full of Legendaries, for one thing. But not just that -- Kyurem is already the "villain" Pokémon, while Keldeo is the "hero" Pokémon. The other three Musketeers also appear. Genesect should be expected next year.
* A new forme for Keldeo; if BW's data (which foretold about one Kyurem forme) are anything going by; unlikely. That said, I wouldn't say this is impossible, although nothing calls it.
* A Generation VI Pokémon; not impossible. I think one can assume that next year will probably bring Generation VI. With 3DS being 2.5 years old by that time, that would only make sense. Generation V came pretty abruptly (which reflects on the games' detached nature from the previous games), but this doesn't have to be the case again. A Munchlax-like Pokémon could be revealed as soon as now.
* The Original Dragon; unlikely, but by far my favorite scenario. I say unlikely because, from what I understand, Zekrom and Reshiram will not appear on the movie, so, Kyurem couldn't stand on his own standing next to or transforming into the original Dragon. That said, if fusions are what we're talking about with Black and White Kyurem, we might see the original dragon in B2W2 nevertheless.