I wonder whether the post-passivation layers (other than the redistribution metal) are transparent. If they are, then instead of adding boring repeating patterns to meet metal density rules on the metal layers, one could include computer-generated hologram patterns instead, so that when a laser pointer hits the die between the solder bumps and redistrubution metal, it produces a diffraction pattern that shows some image projected onto the wall, e.g. the url of the git repository, an OSHW logo, or a picture of a cat, etc.