Dear FeatherofSun,
The loot system can be a bit insane and unforgiving at times, and I'm just talking about random loot atrocities (+1 Tempestuous light crossbow of a Thousand Suns, anyone?)
I am going to propose you a solution, Feather. One that I hope at least gives you an idea that would help create more useful random loot without inasmuch changing its rarity. The idea? 'Legendary' shards.
When loot is generated in a chest, at times, the generator often decides that a weapon will drop. It generates the type of weapon, then assigns it random applicable abilities at the CR of the chest or thereabouts.
My suggestion is to occasionally (a non-trivial percentage of the time--I'd actually prefer it to happen often) have the item that is dropped be a legendary shard. This is a +5 potential shard with all the abilities of a weapon which can be inserted into a Cannith Crafting blank which has no other shards installed. The shard would have a minimum level stat which would be transferred to the item it is inserted into, allowing the item to be crafted for the appropriate character. These shards would not bind (though the finished weapon would be BtA or BtC) and could be traded on the market, further stimulating the economy.
Example:
Old System: Fred the Barbarian finds a +5 Holy Silver Dagger of Greater Evil Outsider Bane. He cries a lot because IT'S A DAGGER. Such a rare combination on a useless weapon!
New System: Fred the Barbarian finds a +5 Holy Shard of Greater Evil Outsider Bane. Excited, he goes to the auction house to find a silver greatsword. He buys it, disjuncts it, and merges it with his new shard! Viola! He is the proud owner of a devil killing machine!
"But" you say, "how do we handle weapon specific prefixes and suffixes, like keen, impact, and precision? Easy enough. Based upon the combination of abilities, limitations as to which weapons these shards can affix to would be created.
Well, it's just an idea, but I think it'd improve game play and generate excitement over random loot once more.