I just made a GRUB splash image using the "Simple Human" background. I've removed the gradients so it looks pretty clean even with 14 colours. IMHO this would be a nice thing to have by default, and I've chosen something that blends well with the rest of the OS.

Attached the xpm.gz (of course), a PNG, and the modified SVG used to generate it. Installation directions here

human-splash.png
human-splash.xpm.gz
human-ultra-simple.svg