This was an idea that I came up in a discussion about a +1 lawful, chotic, holy, evil sword. I suggested this as an alternative enchantment for characters of neutral attitude.
Alternatively, perhaps there should be a neutral weapon. +2 or +3 enhancement, deals an extra 1d6 damage for each step away from true neutral. e.g. chaotic neutral would be 1d6 damage, but chaotic good would be 2d6 damage. The wielder must be no more than 1 step removed from neutral (NG,NE,N,CN,LN) or suffer 2 negative levels.
It shouldn't be more than a +3 as written. Max damage is 2d6 to any creature and only bestows negative levels if you're not neutral at all.
Or you could make it a +4 enhancement and have it do 2d6 per step removed from true neutral. That seem fairly balanced with the holy,unholy, lawful, chaotic enhancements.
Really, it would be no more effective than, say, a flaming, shocking, sonic, acidic sword. That's a +4 bonus and will ALWAYS do an extra 4d6 of damage. The neutral enchantment will only do 4d6 if you're two steps removed, otherwise 2d6 or nothing. The potential for negative levels to the wielder at the 4 extreme alignments should just barely counter the change in damage.