In fact, the company revealed the release date (spring 2015 in Japan) of the game and also what it will be actually about.
Just as previously hinted at, Monster Stadium will focus on PvP, namely 4 against 4 battles and time attack as central game elements. (In contrast, the original is also focusing heavy on the multi-player mode but just lets players cooperate, not battle against each other.)
The way it will work is that Monster Stadium players will need to choose a stage, put together a deck of monsters, become part of a team of up to 4 members, and then clear the stage of monsters before the other team does. (Monster Strike, however, offers no element of time.)