Four SSD drives
Quad density drives: one bay can hold four.
Display with USB 3.1, Multi Card Reader, Fan Speed, and Voltage/Amperage LCD
Voice, music or data.
Turn your computer into a tiny dresser.
Cup holder with lighter
Coffe cup holder and cigarette lighter.
Monitors and controls multiple fans.
With watercooled systems you can hold cooling fluid here. Perhaps if your computer doesn't run exceedingly hot you could use it for a small aquarium. A very small aquarium.
Hot swappable drive bay
If you need to swap drives regularly this works.
EZ bake oven.
Touch screen display