No soldier would be caught dead without a knife somewhere in their BattleSuits. Minifigs will hide knives in obscure places just in case a Brikinid Hive Fleet tries to eat the planet. Knives are commonplace among most Minifigs. Here's their stats:
Damage: (size of creature) d6-2
Minifigs cost one CP less if they don't have a knife hidden somewhere for desperate measures. All minifigs, officers, and heroes carry one unless said otherwise. This could be the answer to the whole "unarmed combat" thing. If all minifigs carrry a weapon somewhere that costs nothing, then there would be no need to have silly rules for fists. If you want to add the knife ability to a creature, you may add it for free as long as the creature has a mind. In other words, if you have a custom unit for your army, they can probably stash a shiv somewhere in their armor or they have claws.
You don't even need a piece for a knife, as it would be assumed that everyone has one, you would not need to keep track of it. Knives may only be used if the minifigure has no other weapons to fight with.
Good idea, bad idea? I wantses yours opinionses.