I love tetris, I mean who doesn't. There are a lot of multiplayer tetris sites out there so here is what I am thinking;

You download a client (or it could be web based) where you can buy in at different stakes untill you get faced with an opponent.

The owner takes a fee the same way poker sites takes rake.

Different modes, Tetris attack, normal tetris (the one who lasts the longest wins), tetris with very fast raising levels etc.

Level up system to get more people hooked - you get progress on your level if you win or lose, but when you win you get more progress than when you lose.

These are just some suggestions comming up from the top of my head, will propably figure out more. If this happens I would like to be involved in some way