Party Time!
The Half-Life world is a dark and serious place... most of the time. What I want to do is to bring out the fun and humor in this game and turn it into a party game. For example, I want to replace enemy npc's with pinatas that spill candy instead of blood. Instead of guns making their normal sound effects they would make "noise maker" sounds. Grenades would also explode with colorful confetti.
Shoot 'em Up!
Another idea I have is to create an old fashion shooting gallery like the ones you used to see in arcades. The player would be stationary except for aiming. the stage would be set up in front of him/her and the targets would move horizontally across the stage. Shooting the targets would yield points, whether positive or negative according to what is hit. Once the ammo clip is spent, the round ends.