As a minimum, constructing an Item of Power is an on-screen event (at least for the first time for any given item), as opposed to foci and normal enchantments that go on in the background...
Actually, I would likely handle at least part of the actual creation off-screen, as it would involve rituals, etc. However, I would definately have the climax of creation occur on screen. Additionally, the Item of Power creation would also have to occur over an entire story arc, the equivalent to a Major Milestone, or two for that matter. All the while, the focus of the story arc itself would be gathering what would be needed to create (or empower) the Item of Power.
Lastly, I would require that the caster(s) involved in creating the Item of Power have to permanently sacrifice the number of Refresh points required to gain whatever powers the Item of Power has, without getting any price reduction for it being an Item of Power. Without making the creation of an Item of Power cost the same as just gaining the power normally, it could lead to players abusing the ability to create Items of Power, instead of making it a sacrifice to do so.
From a perspective on the series, just how many Items of Power have appeared? The four Hexenwulf belts, the three Swords of the Cross, the Shroud of Turin (which might be an Item of Power...), a noose-tie worn by a sinister ~2,000 year old man, a black magic staff sometimes used by a crotchety old Scottish wizard. Plus there might also have been one or two other, additional items I missed. In short, these things are not exactly all that common.
What might be better for players (or more interesting) is for players to have to search for lost/missing/stolen items of power. There are both historical examples of things which might be Items of Power, like the Spear of Destiny, or the Sword of St. Joan of Arc, or examples can be taken from fairly recent history thriller-movies like National Treasure, or the Indiana Jones-series.
