The following stuff was once considered by me, but I went with something else/decided I like the original rules.
Invisible is probably too good. Perhaps halving the bonus?
Even Less Invisible (likely to be implemented)
Or, perhaps sort of basing it on Blindness and concealment? After all, to a Blind person everyone else is invisible... E.g. only a -4 penalty to perceive Invisible character if it's moving (-8 if not moving), but a 50% miss chance on attacks (as the character is considered to have Total Concealment.)