Make a gun (or perhaps add this to all guns, or maybe have a passive skill that adds this to all guns) where you can hold down the shoot buttons for up to 3 seconds and the gun will do more damage the longer you hold it down. Releasing it will shoot the bullet. Each gun could have a min and a max value for damage. You could create more gun variation this way too, for example guns with smaller minimums and higher maximums. For example, whats best for this level? A gun that will do 100 minimum and 150 if maxed, or one that does 120 to 130 damage. You could also have some gun(s) that have insane ranges for high risk high reward like minimum is 10 damage and maximum is 500. It would create more variation and make each gun unique, would create more strategy too.
Discuss?