As it says on the tin, create and/or change anything regarding competitive pokemon in Generation VI.
I should say that one basic rule applies:
Make the additions/changes logical. Don't just slap Tail Glow on Gardevior for example.
And to start things off:
Flavour text: "An item to be held by a Pokémon. This offensive plate raises Defense but prevents the use of status moves."
In-Depth effect: Raises the Defense stat of the holder by 50% (aka +1 increase) but requires the Pokémon to only use damaging attacks.
Comments: A physical counterpart to the Assault vest. 'nuff said.