I just remembered, I was once struck by an idea for a music-themed game that I may as well try dragging out again. The inspiration for this game is parts Touhou Project, Audiosurf, and Vocaloid.
The idea would be to create a top-down shooter that creates levels for the game based on musical tracks that you provide to it, somewhat as Audiosurf does. The game would attempt to create unique levels using a large library of art, bullet types, character types, and so on. You would first need to drop a song into the game to be analyzed* in various ways, during which the game would create a level file that could be accessed at any time afterward. The level would then be played with that song as the backdrop.
Analyses would include:
* Tempo analysis: Analysis of the speed of the piece. When the tempo is faster, enemies will move quickly and thus be more difficult to hit. When the tempo is slower, enemies will move slowly and be easier to hit but will have more health to compensate.
* Spectral analysis: Analyze the balance of low, middle, and high frequencies. When low frequencies (bass) are dominant, enemies will be larger and thus easier to hit but have more health to compensate. When high frequencies are dominant, enemies will be smaller and thus more difficult to hit. Mixing of different sizes will be common.
* Amplitude analysis: Analyze the loudness of music relative to the maximum possible. When the music is loud, bullets fired by enemies will be smaller and move more quickly. When the music is soft, bullets will be large but move slowly. "Large" may also indicate longer laser-type effects or other variations. This analysis will also be combined with the spectral analysis - songs with quiet bass will still have occasional large enemies, but they will fire slow bullets to match the quiet of the bass.
* Pitch analysis: Analysis of the root pitch of the piece. This will be added mostly to provide variety. For songs in the key of C, enemies will follow patterns and shoot bullets common to classic shooters. The key of B flat or G might grant enemies that shoot "lasers" rather than bullets and follow somewhat abberrant patterns, while the key of F# might provide very chaotic, erratic enemies.
* Scale analysis: Analysis of the second and third most common tones other than the root to provide superficial differences. For example, the scale of the piece might set the player in a factory environment fighting robots instead of an ethereal environment fighting elemental sprites.
We want the player to be able to complete the entire song (I hate getting cut off before a song ends), so the player cannot be killed during the game. Instead, hurting the player will daze it, preventing it from attacking and slowing its movement for a time. The main penalty is that enemies might get away while you are prevented from attacking. When a group of enemies (or single large enemies) is killed, it will drop a power-up that boosts strength, size, or frequency for your main or secondary weapon or provides additional "bomb" uses, so the player must try to stay unharmed while staying in line to attack targets.
The actual goal of the song is to defeat the boss. The system will attempt to find the point where the last verse or chorus begins in the song and place the boss at that point. The boss will appear with a timer showing how long until the song ends. If you haven't collected enough power-ups during the level, then you may be unable to deal enough damage to the boss before the song ends.
The player characters will also be able to customize from a number of audio-themed weapons. Each main weapon has a distinctive sound that sounds similar to an electronic music box. Firing your secondary or temporary weapons alongside the main weapon will alter its tone - if you have the Distortion secondary weapon and the Delay shield, your main weapon will sound distorted as you fire it, while the background music track will be echoed when you throw up the shield.
Instrument - main gun:
Arpeggiator - Fires many small bullets in a wide, fanning cone. Gun sounds like an arpeggiated synthesizer.
Drum Machine - Fires large, single bullets in a forward line. Gun sounds like random drum effects.
Oscillator - Fires a sine-wave laser with weaker damage but a wide horizontal breadth. Sounds like a sine-wave tone, transforms into a square wave as it powers up.
Sampler - Fires a thin laser with rapid firing rate. Sounds like various wind or string instruments sampled and looped together.
Effects - side guns:
Distortion - Fires a wide, medium-powered wave ahead of you that covers a 60-degree arc but only travels a short distance away from you.
Flanger - Fires a moderate damage laser that changes direction according to the direction that you move in (moving right rotates the laser toward your 3 o'clock side).
LFO - Fires strong single bullets at a very slow pace.
Modulator - Fires two very weak, rapid-fire lasers at 30-degree angles forward.
Destructive Effects - temporary offenses (bombs):
Chorus - Temporarily adds a "shadow" to your main weapon that increases its size by a fixed amount.
Feedback - Creates a "shadow" of every enemy bullet on the field, which reflect back toward the screen and hurt enemies instead of you.
Overdrive - Charges and fires a single powerful bolt straight in front of you.
Vocoder - Temporarily creates a wave of damage in a sphere all around you that deals strong damage.
Mastering Effects - temporary defenses (shields):
Delay - Freezes the bullets in a large sphere around you so that you can navigate around.
Filter - Destroys all enemy bullets in a sphere around you.
Limiter - Puts up a shield that persists around you for a moment. Bullets inside the shield are immediately shunted to outside of the shield, and no bullets can enter the shield while it is up.
Reverb - Bullets in a small sphere around you are reflected away, damaging enemies if they are hit by the reflections.
** Here's a couple of samples of analysis software that were quickly googled: