Sorry, I know people who are used to classic Tetris will want a line piece! Just like people coming from modern Tetris will want a hold piece, or people coming Puzzle Fighter will want a bomb piece. I know these things would make the game a lot easier. But, with practice the game is easy even without these.
You have a squish move, and you can use it in some clever ways which are even more powerful than a line piece! But, it takes practice to figure out -- and practice to not dig wells in ways that you can't squish down to them.
I'll add a restart button for practice mode and story mode!