Ooh, this ought to be a good one.
I have the standard HHKB layer, and since mine has LEDs I've programmed "H" to brighten ("higher") and "J" to lower, "K" to kill the lights (or turn 'em back on) and "L" to change the LED settings.
As for arrows, I have them as the HHKB placement, but also on ESDF since I use that more than the others. "A" is Home and "G" is End.
Might not be for everyone but it works for me. I do a lot of word processing.
I can post a pic later. Would like to see others to get inspiration; I'm always down for change.