According to a Japanese Shonen Jump scan obtained by Siliconera, it's called Final Fantasy Explorers, and it's a multiplayer action-RPG.
The game will focus on searching for crystals and battling Summons, Monster Hunter-style, in parties of four. Multiple jobs will be available in-game — four of which, the Knight, the Monk, the White Mage, and the Black Mage, are shown on the scan — and taking down Summons will require teamwork and a proper use of class abilities.
Could this be the mysterious 3DS game Nintendo was teasing for tonight? We'll see. In any case, the scan doesn't mention a price, a release date, or whether or not Explorers will be coming West.
Shonen Jump Scan [Ebitsu] Final Fantasy Explorers Announced For Nintendo 3DS [Siliconera]