It's not really that much help, but here goes an idea:
As Excalibur (eeexcalibur #Soul Eater Feelings) is currently made of Fire, Lighting, Ice, Darkness and Poison stones, maybe it's a good idea to make a circular item type.
As some of you may have played DotA, a good example of a "circular" item would be Power Treads, which you can change to Str, Int and Agi.
So new Excalibur could be a 6 in one sword (normal + 5 elements), and every element would give a different bonus. For example , fire could have a stacking dps and frost a chance to frozen the enemy in a hit and so on.
This would make Excalibur at least make sense to the 5 stones. And also, it would be good to make a orb effect to the currently chosen element (fire element = fire orb. and so on)