Wow, good stuff. Gonna have to work my way through the archives sometime.
The "Tetris 2" released outside of Japan is actually "Tetris Flash", more of a spinoff than an actual sequel. The real sequel was "Tetris 2 + Bombliss", which is actually pretty awesome and a game I definitely consider a worthy sequel.
The "Tetris 2" portion of the game is the same mechanics of Tetris, but with an extra mode. The "Bombliss" portion is a spinoff where you have to make bombs and try to clear the entire board. It was released by itself for the Gameboy outside of Japan as "Tetris Blast".
It's got a battery backup so you don't lose your hi scores or your progress through the puzzle mode of Bombliss. The music is also grade A stuff, composed by Koichi Sugiyama of Dragon Quest/Warrior fame.
Yeah, though, Tetris Flash sucks something awful.