I like the HHKB Pro 2 like it is. I particularly like the 60% form factor and highly efficient layout.
However, lately I have been using a RF 87u 55g, and I prefer the sound and feel of the RF to the HHKB. Therefore, if the HHKB Pro 3 would keep the form factor and layout or the Pro 2, here are some things that might make it even better:
+ 55g Topre switches.
+ Reduced cost of Type-S.
+ Address the reported "whistling" problem with Type-S.
+ Steel plate.
+ PBT spacebar.
+ Possibly more keycap choices, but keep the Topre stems. I don't need or want mx stems.
+ Possibly a Bluetooth option.