You might have figured this out given the mahjong pieces, but the teaser site Cave opened yesterday was indeed for a mahjong game. The publisher shared the true identity of the game today: Mahjong Rokumeikan.
Mahjong Rokumeikan is a free-to-play browser-based social mahjong game. Outside of playing mahong with other players, you'll also collect and trade funds. Cave will also be sellingpaid items, although it has yet to share specifics.
Mahjong Rokumeikan will be available for play this Fall. Cave will be holding a closed beta test in advance. Interested parties will find more details at the official site.