Scream at your game controller to mess up the other player
Here's another one of my half-baked ideas from quite a few years back. This one could be the basis for a particularly crazy two-player game. Games are not my thing, so the likelihood of ever acting on it is slim. I put this forth to the world to see what might happen with it.
First, start by visualizing a waveform view of an audio file.
The usual sort of view here is that time goes out to the right, and it grows away from that zero center line depending on the amplitude. Now, imagine using that as a terrain generator. You have some little character who's running that same direction. It's a forced scroll situation, so you can't dawdle, or you will surely die.
The trick is that the waveform/terrain you have to traverse is created by your opponent. How do they make that waveform, you ask? They do it by whispering, screaming, bellowing, talking, or making any other sort of noise into their controller's microphone. The crazier their sounds are, the more complicated the terrain gets.
The good part is that you can do it right back to them. The sounds you make are used to create their terrain, too.
My original idea was for some kind of device with enough space to show your own character while also showing your opponent at the same time. It would need a d-pad or joystick and a few buttons, plus a microphone and speakers. It would also need some kind of way to hook to another unit for multiplayer mode.
I later found out that the perfect hardware now existed for this: the Nintendo DS. You could play on the bottom screen while your opponent fumbled around on the top screen. It was a great coincidence.
In any case, what I like most about this game is the scene it would create. Imagine a couple of kids standing outside, going head to head with their little game systems in hand. They're going to be trash-talking each other anyway, but now the audio content of that blather actually influences the game itself!
Maybe there would be challenges or goals where you make specific sounds at just the right time to gain power-ups or something like that. Imagine the pitch-sensing stuff of the Rock Band games when you play on vocals. You either hit the note and really hose your opponent, or you blow it and fall on your face.
It seems that Nintendo has since added accelerometers and gyroscopes to their newer versions of this device. Given this, having an added thing where you have to tilt this way or that to compensate for some craziness thrown at you by your opponent would also be interesting.
Basically, the players enjoy themselves with the actual game, but everyone else who's just nearby watching them also get to see a great impromptu show. How's that for fun?