There are only two potential improvements I could imagine for my HHKB Pro 2. One is available for purchase, and it would be the silenced version, called the Type-S. It seems expensive, but doing a "silence mod" on a HHKB involves dismantling the keyboard and installing dental bands or landing pads on the sliders, which changes the key travel somewhat and does not employ the closer tolerances of the Type-S switch. In the end, I think it would be worth it to spring for the Type-S. The other modification I think I might like is to have 55g switches instead of the 45g in the standard keyboard. The heavier switches are not available for purchase, and so one would have to open the keyboard and transplant 55g domes from a 55g Topre keyboard. However, not everyone prefers the Type-S or heavier switches. All things considered, I would recommend either the standard HHKB Pro 2 or the Type-S.
Other considerations include color and printed or non-printed keycaps. The standard HHKB Pro 2 is available in 4 combinations: white or black, each printed or non-printed. The Type-S is available only in white, but in printed or non-printed keycaps.