I don't think they will ever release a skin for Dimensionals (except Arthur) as they are heavily time limited units. They can't distribute the character again or the skin, unless they renew the license with the Overlord crew. And even then, the skin would be only distributed for a few month, until the license expire again. So no chance to buy them later, like other skins. Making a Dimensional skin would cause a fair amount of negative feedback too.