I'm assuming everything will be just like it's right now, just the same when you change items clicking the slot you want. Imagine you have a invisible bag where there are a copy of all your legendaries. You can't access that bag, but in character window, you can equip, customize, etc, including the skin. Each character will have their copy, so you can change skins without affecting others copies of the same item. On sigils and runes, it's the same. Ex. Do you want to use a full set of runes? You need 6 legendaries runes. Do you need to use two legendary axes? You will need 2 frostfangs ou a frostfang + astralaria. Remember the objective is to avoid the use of bank to change items from chars to chars.