They're not going to advertise two games at once. The Johto starters appeared in the third DP movie. All the current merchandising and the movie point to Kyurem.
Also take this into consideration. By the time of movie 15 only two Unova legends will be left to debut. Meloetta and Genesect. Both most likely debuting individually from each other. So one legend per film, that's never really happened before (except probly movie 3 & 6). And since anime promotes the games, Hoenn Pokemon may find their way into movies 16 and/or 17. Movie 13 is an example of this.