here’s an expression that could work for opacity.
This, plus 3D stroked lines and a nice glow is about as good of a neon look as you can get.
The credit for this expression is not my own, I picked it from a site, most likely Dan Ebberts. Here it is:
probability = 6;
// “opacity_normal” is the value set for opacity when the a random
// opacity is NOT chosen.
opacity_normal = 100;
x = random(probability);
if (x <= 1) {
opacity = random(100);
} else {
opacity = opacity_normal;
}