A Model M comes pretty close.
But my dream board...
It should have a tactile feel comparable to that of a beam spring keyboard. If possible, though, it should also be quiet.
It should have a long lifetime. Perhaps a reed-switch keyboard, with the tactile feel supplied through ball bearings and magnets instead of springs.
The direction of thrust when pressing the keys should be downwards when the keyboard is inclined a moderate amount (10 degrees) rather than perpendicular to the keyboard base or printed circuit board. This means inclining the keyswitches at the base rather than using tilted stems.
The legends on the keys should be centered, as on pre-IBM PC keyboards (or the present-day Poker II).
The keys should be color-coded with clear functional groupings, taking traditional keyboards as inspiration.
Thus:
black printing on white - the numeric keypad
white printing on green - back space, Enter, Tab, ESC; keys that would generate ASCII control characters on a terminal
white printing on gray - the printable characters in the main part of the keyboard
white printing on black - special function keys (Insert, PgUp, Home)
white printing on brown - the function keys
white printing on red - the break key
white printing on blue - +-*/ on the numeric keypad
white printing on yellow - the four cursor keys
black printing on cream - Shift, Ctrl, Alt
black printing on light green - Fn (if used)
Both shift keys and the backspace key must keep their locations as found on the Model M keyboard or an electric typewriter.
I would only leave room to the right of the P key for one other key, presumably | \, so as to have a big Enter key, like on an electric typewriter with 44 keys.
So I would put the {[ and }] keys to the left of !1, putting }] where ~` is now, and I would put ~` under {[, to the left of the Tab key. This does put these little-used keys a bit further out of the way than they formerly were, but the Enter key is used often, and I think that the improvement from making it bigger is worth it. (However, I admit that this rearrangement is something of an impossible dream - what I really would want is an arrangement like that of the Radio Shack Model 100, for APL compatibility, but that would mean changing all the keyboard layouts in Windows.)
Note that I don't say if it's 60%, tenkeyless, or full-sized. Each such keyboard has its purpose, and I wouldn't mind having my dream keyboard in all three versions.
Oh, and it should be programmable. Not just in terms of the functions of other keys, but in terms of scan codes. Of course, that's a bit more complicated for a USB keyboard as against a PS/2 keyboard, and today a PS/2 keyboard is of limited usefulness.
