So from my understanding, in the elder scrolls series has had daedric artifacts appearing and vanishing in their world. Some are fun, some are powerful, some are convenient. I have thought of a way that I'd like to see them implemented and a way to balance them.
So first off, they would be temporary. This is because there should only be one on the server at any given time. It can have a half hour or hour long bound time. Once one dropped for you it would be removed from being droppable in the future for you. This is not to say that you can't get all the drops over the course of your character's life. They would be dropped from random enemies, randomly gifted, or sent for certain occasions. For example, Wabbajack staff could be fully random (seems apropriate). You may be sitting around crafting when you hear him whisper to you "entertain me or I will turn you to a sweetroll and feed you to your mother!" then it'd appear in your bag.
Now since this game should try to have fun they should be powerful. Like 5 levels above legendary powerful. Azura's star grant you soul trapping with all kills, maybe even giving you free soul gems every 2 minutes. Ebony Mail giving a poison aura and high fire and magic resistance. Wabbajack dealing random damage of random types, transforming enemies, healing them, instantly killing them, or even turning them into items like food or drinks. Skeleton Key giving you 100% force chance. Et Cetera.
I firmly believe that these shouldn't be restricted. Let people really have fun with it. Let them run into cyrodil with the blade of woe and randomly instant kill people. Blade of woe, umbra, sword of jygglag, etc. There is an extensive list of artifacts to pull from. The key is to make them accurate, make them fun, let people abuse them. They will only have them for a half hour or so before they are gone forever. The gods plucking them and sending it to their next hero. Don't let ESO miss out on the fun of the daedric artifacts. (heck have achievements for them with dyes and titles linked to them. Daedric Artificer or something and a nice dye)