My first game will be a 2D platformer game (probably only a flash version).
The character will be controlled with the arrow keys, and the mouse (for the shooting part).
The world is made of cubes, and some parts can be collected with your special weapon (actually I think of a modified vacuum cleaner) and dropped off to access some place not reachable. The world will be modifiable but only at some point. The player will be able to collect only special cubes, and drop them off on special places (it's closer to a puzzle game than a sandbox game). The player can only move one block at the same time, and can't use a weapon during this time.
I intend to include different types of monsters (that have basic movements, can shoot, fly...) and probably NPC for the story part (and why not optional quest).
For the moment (and I think it's going to stay) I will use the lostgarden's Planet Cute PNG. It's been too long since I wanted to use it in one of my games ^^.