I'm also thinking of having a Scoring mechanism, so people can post their scores and see how well they did. I'm going to make it an endless game, and the difficulty just increases the longer you survive - I think.
At the moment I just have a single background image, but I'm thinking if I have time I'll make groups of zones (each zone just has a different background, and maybe different images for the enemies). But that's all time dependent. The key is to get the game playable, looking good and being fun. I can add extra fluff at the end.
No screenshot this time, but there will be (or possibly a little movie) at the end of the day.