I'm not sure if this has been done and I've just not found it or what, but here is the basic idea:
Every type of weapon (Daggers, Bows, Swords ect.) could have a craftable or findable version that gets stronger each kill you do with it.
The original Ebony Blade starts out draining 10 health and after killing 10 friendly npc's, it will drain 30 per hit.
The way I could see this working would be instead of draining health, it does extra damage and raises the damage every 20 kills with the weapon (Against anyone, not just friendlies like the Ebony Blade) The catch being, every weapon would level up differently and start out extremely weak as to add challenge.