Get your grains of salt ready, this is all based on rumors from a NeoGAF thread that claims a verified source at Nintendo leaked this info. A NeoGAF mod has backed the poster's claim, but you should by no means assume this means it's all fact.
Supposedly, the NX console (with the new Legend of Zelda in tow) will launch during the holiday season this year. What other games will launch with NX remains unknown, but the leaker did provide a list of 3DS games for the year which includes code-named titles such as "Caviar F," "Torte City," "N-Stars," and "Pokémon Niji," which translates to rainbow in Japanese. The leaker warned that rainbow is just a codename and does not reflect the content of the game, but obviously the internet is stoked about the prospect anyway.
Rumors of a Pokémon Rainbow game have been around for years, but a game celebrating previous generations (hence the rainbow theme) would be well timed for the 20th anniversary of the series.