Let /clone Only Replace Air Blocks (Not Structures)

It would be super helpful if the the /clone command could clone blocks like grass or terrain only into air, without overwriting existing builds like houses. Right now, using masked skips air blocks from the source, but there’s no built-in way to avoid replacing solid blocks in the destination. This often causes grass or dirt from cloned terrain to replace parts of already existing structures.

What I'd love to see: A new option like:

• /clone <area> <destination> masked replace_air_only

or

• /clone <area> <destination> masked filtered_destination air

This would mean: “Only clone into empty space — don’t replace anything already built.” It would make cloning natural environments around buildings way safer and more creative-friendly.