The patent was originally filed in March and published in late September, then spotted this week by Siliconera.
The GameBoy cover seems to attach like a folio-style phone case. Close the cover and you'll be able to see the screen through a square cutout. Underneath the cutout are buttons that interact with the touchscreen and act as controls.
Paired with some sort of downloadable software, this patented cover could give you a GameBoy-like experience. Best of all, it could potentially work with hardware you already have (like a smartphone or touchscreen device).
Nintendo didn't immediately respond to a request for comment.