Pokémon XY The Movie - The Cocoon of Destruction & Diancie is coming to Japan in July, and CoroCoro magazine (via Serebii has announced that a free 3DS eShop game will be released to tie in with the film. The Band of Thieves & 1000 Pokémon is due on 5th June in Japan and will only be available for a limited time, utilising StreetPass as its main mechanic.
The thieves in question are based on Riot and Marilyn from the movie, and the player's job is to collect Pokémon by getting StreetPass hits from other players; the more 'mon you have, the better the odds of success in battles with the thieves. It sounds primitive, but as a free game to promote a movie and encourage StreetPass usage it could certainly be popular in the Japanese market.
There is hope for the West, perhaps, as just recently it was confirmed that the movie will come to Western territories in 2014, with the amended title Pokémon the Movie: Diancie and the Cocoon of Destruction. When approached by Eurogamer, Nintendo UK issued its standard line that it has nothing to announce at this time. Interpret that as you please.
Would a 'mon themed StreetPass game, referencing the movie, be a download to keep you interested?