The nature of the skins themselves will not change, they are still purely cosmetic and optional. However, purchase of the Warrior Set will help us fund further content and events like this one.
As said by Bigfoot, they're not changing how skins will work other than increase drops.
As for the mystery box, it's possible that the mystery box is connected to the upcoming trader. They're not going to change it to pay2play (other than the Warrior sets for each character which might be in a different gift box)