To those saying Meloetta: The chance of it being Meloetta (and Genosect, of course) is literally 0% - unless they, for some insane reason, decide to include both Meloetta and Keldeo in the film (which they won't). If they were to reveal Meloetta, when they inevitably gave it out, it would leave an open space in Keldeo's spot in the Pokédex. People aren't even supposed to know these Pokémon exist, so they give them out in order so they don't spoil the existence of any more until the time comes. It keeps people wondering if there will be more, or if that's it. Hidden Legendaries are a huge draw for the games and anime, and they make a lot of money and hold a lot of fans interested by keeping them hidden.
The reason there are two question marks are, if I remember the patterns correctly from previous films, they represent two different images of the hidden Pokémon. Different stock images for the products.