I really think that the "Starz0r" example is a good idea
.
But , well ... i think it should be fun if the player press a key to shoot the enemies in rhythm .
For example, the enemy always throw bullet in all the map according to the beatmap ; and the player do the same but only on a vertical line , but, few time the player must press a key to make a powerfull attack (indicated by something like ... color change)
I hope that you understand what I mean
( also , i don't read all pages of the topic, so ... tell me if something better is proposed )