I think adding an all knife round as a fun round would be great.... The insta knife would not apply in this game mode so it can be fair game.
How about Throwing Knife wars, similar to Harpoon wars except it would be different because when you throw the knife gravity enacts on it more than a harpoon. Would be a slight change up to the already popular fun round game mode.
I like maybe a slight mod to this idea, knife "melee-only" mode. Think that would be cool, but the idea of knife round as bonus round overall sounds appealing. +1