what we need is some company to make key caps for cherry key switches, and made from all sorts of material like the rubber from the razer lycosa, wood varnished and matt, metal with etched surface like a maglite, usual concave keys and apple style flat keys, glass, fabric. If this could happen then a more modular keyboard be created.
A full on modular keyboard should be larger than normal though as you wouldn't be restricted to just cable and key caps, but also keyswitches and key layout. Imagine black brown and blue keyswitches of varying strengths within a keyboard and arranged the way you want them to be. Course it would never happen as the cost of such a ultra niche device is fairly limited at best, + cost of keyboards are so cheap it might be cheaper to buy multiple boards.