I think friendly fire would bring a layer of tactics and strategy to the game that would be fun. We already have friendly fire from explosive barrels. When an ally shoots a cannonball at me while I'm dueling another player, the cannonball should hit both of us! If skeletons with flintlocks shoot at us and another skeleton gets struck by the pellet, that skeleton should take the damage! If someone is swinging their cutlass around wildly it should hit everybody equally!
I think the gravity in the game is weird too when you get knock back from weapons. It's very un-natural feeling and feels confusing even though I know what it is.