If I literally wanted visibility expanding outward from a point-- an expanding sphere-- I would use object coordinates from an empty to mix with transparency.
Something like this, but with transparency instead of black emission:
Then, you can scale the empty providing the texture coordinates to change the size of the visible buble (and, of course, keyframe that scale.)
You can try that. It’s probably not what I’d want in this situation, because your lighting bolts are continuous and jagged, and just drawing an expanding sphere won’t look right.