Run 'n' Gun

So yeah! New games ideas are always fun to explore, and this one's pretty simple - as in totally do-able fairly easily!

Basically, we have a guy. Probably some sort of robot or mega-person. He's on the run from a giant robot/monster, which is chasing him through a very uneven area - most likely a ruined city or some sort of underground military base. Anyways! As he runs, he can shoot back at the big robot, which damages it. After he's taken enough damage, he dies. However! The guy *is* running for his life - the killer robot is armed with weapons, and isn't gun-shy. The guy will need to avoid enemy fire while navigating hazardous terrain and returning fire in the meantime. Oh, and the big monster isn't the only target either...

Okay, so, we control the PC by using WASD+Mouse - W jumps, S slides, A moves left, D moves right, the mouse aims your gun, left-click shoots, and right-click fires your alternate gun, which is chosen before the mission. Or is picked up from weapon-crates scattered about the level. I think I like that one better.

The PC takes damage from colliding with hazardous terrain - things like spikes or lava (real video-game hazards). He also takes damage from colliding with enemy fire or with the big guy. Little enemies don't hurt him unless they lash out at him with a weapon... or some sort of defense system, like spines or a fire shield.

So the primary fire should be a rapid-fire deal - some sort of assault rifle or machine gun. The secondary fire will be whatever weapon you've picked up as you run away - this'll include flamethrowers, grenade launchers, laser-cannons, bazookas, etc. The ammo will be limited for each alternate weapon - once it's gone, you'll revert to a basic melee attack.

The first idea is a guy escaping from a giant robot, but this has merits for expansions. For example, that same guy could be running from a giant ogre, a dinosaur, or whatever. Each "level" could feature a different monster, level-layout, and weapon selection. But let's start with the basics first.

Just some quick weapon ideas:

Machine Gun - fires a steady stream of bullets in a straight line. Default primary gun.
Shotgun - shoots a cone of pellets that cover a wide arc, hitting multiple targets multiple times. Alternate primary gun.
Ricochet Gun - shoots a projectile that bounces off walls, only stopping when it hits a target. Alternate default gun.

Grenade Launcher - shoots a bouncing grenade that explodes on contact with an enemy.
Bazooka - shoots a slow-moving rocket that causes major damage to whatever it hits.
Laser Cannon - shoots a laser that cuts through everything it hits.
Flamethrower - shoots a constant stream of billowing flames.
Lightning Rod - shoots a lightning bolt that arcs off a target and strikes multiple foes.
Missile Pod - shoots a barrage of missiles that home in on the cursor.
Boomerang - shoots a blade that travels in a looping pattern before returning to the PC.
Mine Layer - shoots an explosive that sticks to a surface and detonates on contact.

Also, forgot to mention: The PC automatically runs away from the monster, and the screen automatically scolls to the right, so moving only really adjusts the PC's position on-screen.