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

This sounds fun as long as no money is involved, but how is this gambling? As far as i know, Tetris is 99% skillbased, which means good players will almost always win and bad/new players almost always lose. It's also very easy to bot which makes it impossible for new players to join without being completely ripped off, which will probably make them leave very quickly.
Well you have no idea who you are comming up against so it's a gamble.. There are a few skill based games here in gambling, which forum whould I post in? Did you even read my post about different stakes? $1 buyin, $5 buyin etc, you buyin where you feel safe - the leveling system could make it impossible for good players to buyin at $1.