Regarding the number of buttons, the Space Cadet had 100 buttons and no number-pad , whereas most modern keyboards have 104 buttons. I suppose I could add a number-pad to my design (117 buttons), but then I could also use that area for extra user-definable buttons (20 buttons in a 4x5 grid -> 120 buttons). The Space Cadet is a bit larger than most IBM-style keyboards, so more keys means more real-estate; this is not to mention yet further divergence in design from the original Space Cadet keyboard.
Beyond hardware issues, there are software issues to resolve, like whether to include the macro functionality of the original. I can't find any documentation on how it worked, so I get to start from fresh.
 = I really wanted to use some hall-effect switches, but nobody makes them anymore, because they are allegedly the most luxurious switches ever. I would probably have to tear apart an original Space Cadet keyboard to get some. Thus, I would probably just use Cherry MX switches
 = https://upload.wikimedia.org/wikipedia/commons/thumb/4/47/Sp...