A GRUB theme would be nice, but as @popey suggested, probably not something a lot of people care too much about.
On the other hand, having a proper GRUB theme for EFI boots (like the BIOS ones already have) on the installation media would be really nice, since that’s something everyone with EFI sees when they boot the ISO.
And while we’re at it, what about language selection like the BIOS (syslinux) bootloader has?
Given EFI’s ever-increasing predominance, I see this as something of high importance.
I seem to remember talking to someone (@adconrad?) about why this was the way it was, but I don’t remember the reasoning.