Nice! For a bit more challenge on on-theme-ness, you could add a rule where at least one letter must be kept, or a random letter from the word. (But remove the timer) Was not trivial at first that I can replace existing letter and not just have to extend at the beginning and end, which was hell :). Keeping it at three letters forever howevers makes it a bit easy. (Except for the timer) Not sure that more and more letters being available is intended or a bug, but that should probably be limited, or tied to current world length? For the power-ups, some attention-grabbing animation might be good, I completely forgot about them. Overall great idea, well executed!