We've recently started planning out our Canvas based game; Andy and I sat down to plan out a theme/game mechanics and we came up with the following.

It's going to be a top-down, zombie themed shooter game. The player will be stuck inside a building from which they can't escape. Zombies will spawn offscreen and attack the building, destroying barricaded entrances that the player can rebuild. The aim is to balance the time you spend killing with the time you spend repairing and looking after the barricades. Gameplay will be continuous, with points being awarded for each enemy you kill and barricade you fix. Different classes of Zombie have varying attributes, such as speed/damage dealt. Weapons and power-ups will be dropped randomly.

We're hoping to integrate a high-score/leader board system into the game using a bit of PHP/MySQL magic, which shouldn't be too difficult.

A couple of sketches: