Pretty solid primary gameplay loop. Very smooth controls. I like the occasional reversal of roles, a la Pac-Man.
Note that I had no audio. If you didn't implement audio, consider doing it next time... a little bit of effort can go a long way toward enhancing immersion.
My main recommendation would be to gradually diversify the threats, and maybe even the rewards, over the course of the game. More types of enemies, different kinds of pickups, maybe environmental challenges or hazards...