I'm a huge Tetris nerd, so I'm curious: is this going to be old-school Tetris (eg. like NES Tetris) or a more modern version (eg. like
TetrisFriends)?
Also, I'd recommend you have a few different modes of Tetris. Sprint (clear 40 lines as fast as you can) and Ultra (get as many points in a certain time limit) would be a good start.
Thanks for you comment

In the initial fase I'll not spend a lot of time on game design, I will focus on functionality and gameplay, in particular in challenges and money transactions. Games will have a standard design. If everything goes fine, I'll start to edit the game design, and I'll prefer old-school.
Same for tetris modes: we'll start only with Sprint, and then add all others modes.