First, don't think I'm being pushy here, I'm just tossing out ideas for you. I know enough of what I'm doing I can just change it however I want, but not all users know how to do that. When I used to write mods for other games, some users would get pushy and behave as if I somehow owed them something. I've been in your chair and I know those users can turn the experience sour really fast.
With that disclaimer out of the way:
Here's an idea for you. You could toss in a settings.lua file, put in four boolean options (or however many you want to do) and say the boolean options are 80%, 60%, 40%, 20%. The user picks the one they want and then you just add a quick bit of logic to the lua to change which graphic images it points to. Then you just create a different image for each one with the opacity settings that match the option. You get the idea. Just a thought. You may want to give an option to disable the shadow.
PS: Make sure you put the setting to the write type, because if you use the startup type it can only be changed at the start of a new game, and that'll be annoying. https://wiki.factorio.com/Tutorial:Mod_settings
psuedo code:
if settings.runtime-global["eighty"].value == true then
data:extend(
{
<...all the various settings...>
filename = "AlmostInvisibleElectricWires/graphics/hr-copper-wire-80.png"
}
if settings.runtime-global["sixty"].value == true
<various settings>
filename = "AlmostInvisibleElectricWires/graphics/hr-copper-wire-60.png"
}
