This was awesome! Incredibly addictive. I love the idea, simple yet super effective. Congratulations also on wrapping everything up so nicely.
If I wanted to expand the project, I'd say work the gfx (though what you've done works really well already), and maybe add some more sense of progress. Maybe have 'waves' that you have to complete, and each new wave adds more difficulty, or even better new gameplay elements, such as power ups, special terrain (i.e. swamp tiles that make you walk slower)... I can totally imagine this evolving. :) Great work in any case!