Hm... how about an item that would prevent decrease in stats on the Pokémon that is holding the item? Or maybe even an item that totally prevents status ailments? (There might already be one, but I have a terrible memory of things.)
But most of all, I would like to see items that only work if the Pokémon that is holding it is the main legendary Pokémon of X and Y. Similar to a way that the Adamant, Lustrous and Griseous Orbs.