I think the first thing you have to do is take a long hard look at the wealth system, and how you use it in your game. Do you always give out standard treasure? Do you try to keep near wealth by level? Does your group evenly distribute treasure or is it according to need or some other criteria? Are there magic marts? Do players craft magic items?
I'll fully admit that I am a pretty new GM, so some of this is all still very new to me. The characters started out at 5th level and had the full amount of starting cash they should have for that level. Since then, they haven't really paid too much attention to treasure. They'll ask, but they don't seem too concerned about it. I think with this group, as long as they have the ability to upgrade when necessary, they'll be fairly easy-going about such things.
The setting is actually Eberron, so it's a magic/gear heavy setting, but I also don't want to lose the slightly gritty feel the lower-mid levels can have.
Next you have to ask yourself some questions about the new items. Will all of the players in the party have a unique scalable magic item? Will some have more then one? Will they be near the same 'value'? If there is an imbalance, how will you deal with additional treasure/magic items? How will they be distributed? Will the pc with the ancient sword of awesome get the same share of the treasure as the guy with the sad little flute of uselessness?
The items I'm looking to design would actually be very plot centric. They'd be linked to a specific character and definitely only one per character. The only major issue would be that, being plot centric, they may find one fairly early on, but not find the last one until much later, letting some players have access to the cool item early on, while others have to wait.
Basically what I am trying to say is you need to figure out how these items will fit into your game first, THEN start designing rules to go along with them. If everyone is going to get one that is roughly of equal value and use to the character, then just scale down the treasure a bit and move forward with whatever you want.
That seems pretty reasonable. I've got time to figure out what the items are going to be since they're just getting to the point where they'll discover they need to go looking for the items, but since I've got to set some of the specifics now (like where they are), I got to thinking about how to actually make them work.