I've also been looking to supernatural mystical weapons - such as the Witchblade. That might be good to use as possible slayer weapons. But if a Slayer were to wield the Witchblade - she would be such a hand full that only a very small percentage of Big Bads would stand a chance - against this combination.

The Witchblade is actually just one of several mystical weapons - some serve the forces of good - others the forces of evil. The Witchblade itself is actually a blend of The Angelus - a powerful weapon of light, and the Darkness - a weapon of evil. Hince it's highly unpredictable nature. I use the Witchblade as an example -because the Witchblade, has a "chosen" female wielder per each generation. And is a weapon used to fight the forces of darkness. Which is very similar to the Slayer line.

The best way might be to look to the Witchblade & it's kin as examples - then tone their stats down to limit their power when wield by a Slayer.