Personally I don't think Arceus deserves another item. Technically it already has 16 of them, all of which do wonders when used correctly. Giving it another item similar to Dialga's Adamant Orb or Giratina's Griseous Orb, would just make it an overpowered Pokemon that has the potential to rip everything in it's sight. Sure, it is a legendary Pokemon and the so-called creator of the Pokemon universe, but even then 17 items specifically made for one Pokemon is overdone, don't you think?
The elemental plates already make Arceus a terrifying Pokemon to behold, giving it another Pokemon is just overdoing it. While it would be a good addition for another orb, I think Arceus doesn't necessarily need it. Even if it did exist- Pokemon are only allowed to hold one item- so then you'd have to decide between giving Arceus a completely new type or more raw power in battle. It'd be confusing and yeah, in my opinion Arceus just doesn't need another item at all.