Since I didn't feel like reading 17 pages to see What has, or has not been suggested, some of my ideas might already be suggested.
Anyway,here's my take on it!
Bullet spawningSPOILERHitcircles
-Bullets spawn in the top quarter of the screen, based on horizontal position
-Random amount of bullets (Depending on diffeculty)
-Random bullet direction (full 360°?)
-Varying bullet speeds (margin based on BPM)
-Streams Shoot a bullet aimed at your current position,
Sliders
-Spawns an enemy on the start and the end of every slider (Again, top quarter of the screen)
-Enemies move at a set speed (based on BPM)
-Enemies move randomly throughout the top half of the screen (and go off-screen after a set amount of time (based on diffeculty))
-Enemies have a different attack pattern (and sprite) based on hitsound (random, aimed, in a circle, straight down, ect.)
-Reversearrows shoot a bullet at your current position (Like streams)
Spinners
-Shoots bullets in a spiral from the center of the screen
-Turning speed based on BPM
-Amount of bullet streams and bullet density per stream based on diffeculty
-Fades in so you have time to react (fade-in speed based on approach rate)
Combo / ScoringSPOILER-Enemies that are killed will spawn a proportionate amount of point pickups (that drop down)
-Amount of points dropped is based on enemy type and how long it took you to kill it
-You get a very small amount af points for every bullet that misses you within a certain area around you (area is based on diffeculty and combo)
-Combo builds up when you kill an enemy (also nets you some points)
-Combo breaks if you get hit
-The higher the combo, the more damage you do (incerments whenever you would get a comboburst)
LifebarSPOILER-Drains slowly over time
-Drains when you are hit
-Fills up if you kill an enemy
-Fills up a little bit when collecting points
BombsSPOILER-Bombs spawn in a random location whenever there is a pause moment in the song
-Start out with 1 bomb
-Bombs stack up to 3
-When used, a bomb will destroy all enemys and bullets currently on screen (no points awarded)
ModsSPOILEREasy mode
-3 Lives
-Slower lifedrain
-More life recovery
-Less bullets
-Less enemy health
-Slower enemies
-Longer fade-in for spinners
No fail
-Self explanatory
Half time
-Self explanatory
Hardrock
-Opposite of easy mode
Sudden death
-Can't get hit
Perfect
-Can't get hit
-Collect all point drops
-Kill all enemies before they go off-screen
-No bombs
Speedup/Nightcore
-Self explanatory
Hidden
-Bullets dissappear in a certain radius around your character
-Area increases with combo
Flashlight
-Can only see a certain area around you
-Area decreases with combo
Relax
-Auto targeting
Auto pilot
- Score drops "home in" on you
Auto
-Self explanatory
MiscSPOILER-Accuracy is based on enemies spawned vs enemies killed & bullets shot vs enemies hit
-Everything marked as "random" should be fixed per beatmap/diffeculty for consistany and fairness.
Thanks to everyone who managed to read my enormous wall of text.
I've probably spend way too much time on this then I should, but it was fun thinking of all the game mechanics, so time well spend I guess.
Feel free to criticize/expand on my ideas.