The easiest way to enhance a weapon is to give it aspects, magical or mundane, temporary or constant, whatever would fit your needs.
Give it a maneuver spell that is going to last for a scene (things like "brimming with power", "superheated" or "supernaturally sharp" for example), you can activate it before you go into a fight (or even during a fight, if need be) and you will have 1 free tag on it, and if you need, there will be more available by the use of fate points. And as an enchanted item, you can activate it again for 1 mental stress.
You could give it mundane aspects, too, if for example it was made by the worlds best bladesmith or something like that, a weapon can have a story as much as a character, if your GM is ok with it.
I would never let a weaponrating and a spell hit at the same time. You either unleash the spell on the item or do a weapon attack, even if the weapon needs to connect to let the spell hit. At least that is the way I see it.