I'd love an option that would allow me to replace any air blocks that has no or low light levels with another block.

Not me, but my friends often use world edit and other build tools in a manner that leaves a mess. Random air pockets underground. Mountainous terrain or walls that are clean on one side but mess on the other. Cleaning up these messes without ruining build's shapes can be really time consuming.

When it comes to air pockets, one command selecting the whole map can often solve my issues.

When it comes to walls or fake mountains. I just have to make sure the area I want filled in is properly enclosed before running the command.

I assume it'd be something like this:
//replace air[Light<4] minecraft:stone
//replace air[Light=0] minecraft:stone

I guess this could also be used to clear anything not visible from the surface

//replace minecraft:stone[Light<4] air

Bot sure if that'd work
But it'd let you exclude important blocks or areas nearly based on what's visible.

With the new light block, this would be seriously epic as we can light enviroments effortlessly for noob players who play on the default brightness "moody". Any cheeky chance we could get some progress on this?

Write it yourself :)

This depends on a rewrite of the mask system so it's not something that can be done quickly