Yeah, grapple would be cool. Do you like the C4 idea. If not heres another:
fork ray, it shoots flaming forks, laser forks, and any other kind of fork you can think of at the enemie.
Ooooh, I have a really cool idea... How about the ability to customize the look of your gun. You start out with preset parts, pick the ones you like, and then you add the bullet (power), from a list of bullets you have unlocked. I think that would be sweet.