Some ideas of my own . . .
This really belongs in house rules, but for those of you who may be interested in this topic ...
Mithral See DMG, p.242. Mithral weapons may be fitted with a small piece of sparkstone, which forms an electromagnetic field when in contact with mithral. This electromagnetic field converts the kinetic energy of an attack swing into +1 electrical damage.
Sparkstone See Mithral, above. A piece of sparkstone the size and weight of 2 gp is necessary for the special damage effect. Sparkstone is a rare ore, usually costing fifty times as much as gold.
Kobalt A rare metal with a bluish tint. It maintains its shape and sharpness quite well, and penetrates adamantine like any other metal. Priced as adamantine.
Titanium Steel Slashing weapons made of this alloy can be made especially thin without losing strength, and are thus considered keen. Because these weapons are so thin, damage inflicted is reduced (-1). Due in part to the metal’s lightness and in part to the sleek crafting, these weapons weigh half as much as normal weapons. Priced at +2000 gp.