When speaking of loot, it is good to look at what is expected of it.
The first thing that came to mind is what place the loot can take in Fate to increase the narrative. For this aspect of loot, I came up with a couple of goals.
Loot should add to the narrative. At it's core Fate is a storytelling game. If the hero picks up a random sword, it doesn't add in an overall way to the narrative. However, the Bloody Blade of the Unstoppable Warlord Kharm tells a story, and over time can add to the narrative.
Loot should adapt to the narrative. Over time, as the character changes and adapts because of the story, if the loot that he obtained in his first adventure never changes with the character, it makes one part of him static. Boring. As the character changes and grows more powerful, so should his loot.
The second part is the general player satisfaction. One part about seeing a chest after a long fight in a dungeon is what secrets does it hold, and how will it make me more powerful? If the player doesn't receive something crunchy from the loot obtained, it might fall short in satisfaction for a long road slogged to get it.
The third part goes hand in hand with player satisfaction- balance. If only one of the players is going to get the Bloody Blade of the Unstoppable Warlord Kharm, will he suddenly become out of balance with the other players? Will you start having to balance towards his character rather than the group?
So, taking these in mind, this is what I did for a fate-core fantasy game that I recently ran.
In the Extras chapter in the Fate Core book, the rules refer to Weapon and Armor ratingsFC277. In a game with loot, these are a necessity IMO. They differentiate the types of weapons, and give more mechanical heft to the items found, and their lack.
Once this is in place, I dug further into the extras (and the yet to be release Fate Toolkit), and looked at how I viewed the parts of the game as they related to things that were not a part of the character- namely Aspects, Skills/Stunts, and Fate points- and how they related to advancement.
The one concession that I first made was that the point was to balance in general, and not at any given point in time. After the person picks up the Bloody Blade of the Unstoppable Warlord Kharm, for a short time after he is able to use it, he will be more powerful. But by the time of the advancement, I want them to be on equal or near-equal footing. This also makes the placement of items within the story important.
The other thing I did was treat any weapon worthy of being loot as a Fractal. This was overkill for many cases, i.e. I only want to add an aspect to a weapon... is this necessarily a fractal? But what I found in the long run, looking at my goals, this actually increased player satisfaction with those special items, and over time gave them personalities of their own as they advanced alongside the player.
With that out of the way, the first thing to look at is Aspects. Aspects make a character more flexible within the narrative, and there is no explicit way in advancements to get another aspect- only to rename them or change them around. This either makes new aspects very powerful (in that you don't get more), or more of a narrative issue (in that you can switch them with narrative changes at any milestones excepting the high concept/trouble).
I chose to view aspects as not an issue other than with flexibility, and so associated no cost (other than having the item) with aspects on items. So if the Bloody Blade of the Warlord Kharm has the aspect Hungers for the Blood of Warriors, I wouldn't say that was anything other than a characteristic of the blade. You can invoke this (and it can be compelled against you) only while the blade is in on your person. But other than this limitation, you can use it just as you can any of your other aspects.
The next thing is Skills and Stunts. These definitely mechanically make the character not only more flexible, but also more powerful. The limiting factors to this are similar to stunts - the cost of Fate Points to activate. But just having the ability is an advantage also, even if you don't have the use of it- you have the option to do so.
With this in mind, I treated these similarly to stunts, one and all. I rated them for refresh cost, and if the character chose to keep the weapon, they had to pay for the cost in refresh- either out of their current refresh, or incur a refresh debt to be paid over time (see below for more on refresh debt)
The last thing is Fate points. This is a very powerful thing to put into an item. Adding Fate points to an item is to give it refresh. In the Dresden Files, it made it very clear that refresh was akin to how much free will a character had- so I looked at items that were in this category as items that had their own will, i.e. artifacts. The could use them for the character's benefit... but this is a two edged sword. Again, this is something that comes into play with the concept of a refresh debt)
With that out the way, the last thing I added was something of my own- what I call a Refresh Debt (countered by Refresh Investment).
The more refresh an item costs (either from Fate points or the cost of an ability), the more powerful the item is. It takes a very strong willed person to tame such items- they have to invest a part of themselves into the item to get it to obey their will. Until they have done so, that will is in the hands of the object, i.e. the user can find themselves in an escalating contest of wills with the object. Any Fate that is in the hands of the item is also always in question.
Take the example from above- the Bloody Blade of the Unstoppable Warlord Kharm. It has the aspect Hungers for the Blood of Warriors, it gives a +2 bonus to any Fight attack made in when fighting more than one opponent (1 refresh). It is a 1 refresh item, and costs 1 refresh to the character that will wield it. The character can pay 1 refresh immediately, and dominate the weapon. But what if he doesn't have the refresh?
In that case, the amount of Refresh Debt is utilized in contests of will, and when determining the frequency of them. It is basically used a Fate points that the item can use towards asserting its dominance. In the case that items have their own Fate points to use, this is always counted as refresh debt. The item has its own will at that point and is not just a character in mechanics, but a character in truth, with its own personality, and can be bargained with. In those cases, I also gave the item a mental stress track and applicable skills that could be used in these conflicts.
Refresh Investment is the counterpart- being how much refresh, i.e. how much if the character's free will is invested in the artifact. This takes care of the fact that the artifact is an item that can be lost/stolen. This refresh investment can be used as Fate points in actions to locate and recover the artifact.
The last part of the equation is Advancement. When the character reaches a milestone in which they have been utilizing the artifact to a great extent, the moderator can talk with the player and choose to apply a milestone to the artifact. This milestone can be something that the character and artifact strive towards (i.e. unlocking an additional effect that the artifact has), or something that the artifact itself strives towards (i.e. increasing its dominance and will). If the latter, it should have been something obvious in the narrative, and talked over with the player, and especially evident in the actions that the artifact has taken. At that point, the player can choose to spend the character's advancement re-asserting domination, or let it carry the character further into refresh debt.