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I would love one of these. I would also vote for Matias Quiet Click switches, if being able to choose is not an option.
I suggest you put keys Home and End on the two spaces vacant around the Up arrow. This would free some space for an additional Fn key on the right side, perhaps.
I would put PgUp and PgDn as a second layer of Up and Down, freeing even more space for more function keys, but I suppose your audience in general would not appreciate this weird decision about PgUp and PgDn.
I would also reduce the size of the space bar in order to allow one more function key on each side. Your Japanese audience would strongly agree but probably the rest of the world would find this weird. Anyway (if the keyboard is reprogrammable!) you could provide a longer space bar which simply revert the appearance to the occidental standard.
I'm a heavy Emacs user and I reprogram my keyboards in a way that allows me to reallocate keys so that I avoid excess stress of the pinkies. The thumb is the strongest finger, so I find ways to make the thumbs responsible for pressing Ctrl, Alt and Fn, which are keys you will be pressing all the time when using Emacs in a 60% keyboard, in particular if you have to press Fn+IJKL in order to obtain arrows, for example.
This may be of your interest: http://github.com/frgomes/carpalx