I disagree with the need for an "Item" power. If a Wizard made the item, it's Arcane, if a Cleric made it it's Divine, if a Psion made it it's Psionic or Mental or whatever.
That makes a lot of sense. The only thing I would note is that the 4e power sources tended to define how the powers worked (that is, Martial tended to not get Dailies, post-Essentials at least, while Psionic got those Augments, and so on).
There do seem to be a fairly large number of commonalities between Items of all types (not least that they can easily switched out), and a number of things that distinguish them from the 'parent' power source, whatever that is.
Artificers or similar focussed classes seem generally arcane, so there's no need to have a separate power source for one class that appeared in a single campaign setting.
In addition to the Eberron-style Artificer, I would expect the Item source to cover characters like the Steampunk-style Mad Scientist (or Tinker Gnome, if we must), the character who doesn't create the items but merely uses them (Jarlaxle), and the character who is intimately tied to a single signature item (Elric with Stormbringer).
Plus, by supporting classes in this source, we also get auto-levelling items, Weapons of Legacy, or signature items (Anduril, etc) for free - those would simply become a matter of multiclassing.
The game can certainly manage without a dedicated Item source... but I do think it would be helpful.