Add capability to "Keymap" preference app add and edit custom or additional keyboard
Already there. No GUI, but the functionality is there. I don't think an OS needs to provide a nice GUI for every command line application. I would say: good task for Summer of Code or for 3rdparty developer.
with additional keys (multimedia and etc)
The additional keys (at least for Laptops) doesn't generates scan-codes at all. They managed through EC or I2C controller. To handle them a special driver required and frequently they are BIOS and hardware/oem-specific, so no generic driver possible AFAIK.
capability to scan pressed some key on actual keyboard.
What if they doesn't generates any scan-codes?
I would say a better ACPI support could be a nice enhancement, as without proper DSDT handling you cannot have any additional key-support, brightness control, hardware-switch control, hotplug, power-management, sleep, hibernate, and so on.