Cursed Scroll:
WHY - It follows the trend in 5th and 6th Edition (aka. Jester's Cap and Grinning Totem, respectively).
WHY NOT - Banned in Rath Cylce Constructed. Also, would totally wreck the slow T2 environment.
Coat of Arms:
WHY- Would promote theme decks. Merfolk, Elves, Zombies, Beasts, Angels and Spirits could all benefit.
WHY NOT - One word: Rebels.
Mox Diamond:
WHY - It would enhance the multi-colored theme being promoted in T2, without being overly unbalanced. At least, not with the current T2 card pool.
WHY NOT - It's a Mox. R&D are really phobic about 0cc artifacts, especially those that get banned or restricted in T1.
Scroll Rack:
WHY - Cheap library manipulation. Colorless, too.
WHY NOT - Cheap library manipulation is supposed to be something that blue can do best. Compared to the Scroll Rack, Soothsaying just doesn't cut it as well.
Those are my ideas on possible candidates as "power artifact." Others worthy of being in 7th (ex. Puppet Strings, Torture Chamber, Static Orb) aren't quite on the level of being called "power."
My personal vote? Go MOX DIAMOND!!!