One thing I like to do is to take some aspect of a weapon and look at how that weapon would be different if that aspect were much smaller/larger – and then using magic to explain how it got there or what else it could do then.
A simple example is swords: you can describe swords by how long the blade is, how long the handle is, how sharp they are, how deep the blade is, how easy/hard they are to create, how easy/hard they are to maintain, etc. So you can take any one property (for example, sharpness) and say, "Okay, this is the sharpest sword in the world – who created it and why? What's it used for?" and now you've got the makings for something legendary. A less obvious example could be taking something like a gun – what happens when you take the length of the barrel as a property and stretch it out to 5, 10 feet long? Suddenly you have a super long gun that could also be used as a staff (yet fires bullets), or maybe it needs that long of a barrel because it can fire sniper rounds a thousand miles away, or maybe that long barrel is there to accelerate a bullet 100x the normal bullet speed to penetrate even the hardest of armors.
Play around with every aspect of every weapon and you'll find lots of fun, weird stuff. At that point, your next step is figuring out how magic helps (or hinders) that weapon – whether in the manufacturing of it (using magic to keep steel stable when you stretch it out so much), the maintenance (this sword needs its enchantments renewed each day or it becomes as brittle as glass), the usage (this gun needs complete focus of the mind to fire), or something else. :)