Lethality, Armor Penetration, and Magic Penetration expansions?

I don't know if anyone has suggested this before or anything, but can lethality, armor penetration, and magic penetration be expanded? Can new items with those stats be added and/or can they be added to other existing items, and then set a cap on them like the 45% cap on cooldown reduction? I feel like this would allow people to have more freedom to get what they want in a build, and not limit themselves to a few items and have to choose what they'll add to that with what space they have left.