That was super cute and novel, but I want to point out weak points so the final game will be the best. (spoiler warning?)
Currently it's extremely tedious to clean up a lot of corrupted blocks. I would suggest giving the player a minigun if the player gathered more than 20-30 bullets, or if they cleared N number of lines. Or something just as ridiculous
Some technical stuff like proper game over, pause, (VOLUME SETTINGS) etc. Making gun mode less overpowered (like currently you can enter/exit for free and chip away any tetris piece into a straight piece for example)
And just generally flesh out the game more. Make more use of Flapper. Make corrupted blocks bigger, more menacing, a challenge. Same with the boss, maybe make his sprite/model get procedurally generated based on current board arrangement?