According to an IGN video sent by a friend (I will link to it later, my computer refuses to load the link right now) and this Bulbapedia article, it possible that the next set of Pokémon games may be called Sun and Moon. Trademark logos for said games have been found in a European trademark registry filed under Nintendo’s name.
Keep in mind though, several unused titles have been filed in this manner in the past. Remember Delta Emerald? This does not make Sun and Moon legitimate at this time.
Junichi Matsuda put out a tweet on February 18th that adds to the speculation.
The Sun is still out, but the Moon is big, bright, and shining!! The birds are also tweeting. (Laughs)
Fans speculate that Sun and Moon may be the newest games announced by Pokémon Direct tomorrow. This is unknown, however. More details when info is available.