So right now, neurotoxin only affects players. There is really not much point in having multiple in a deck other than getting one earlier, which is kind of annoying for a spell effect.
It would be neat if the effect could target creatures as well.
A creature with neurotoxin on it would get +1 poison anytime the owner activates its ability.
Simple subtle buff, but adds some utility for dealing with annoying growers, etc...
Could also be very intersting in gravity or voodoo decks.