I think that the most important thing is that it would be serviceable. You should be able to replace a faulty component if possible. Simple, rugged materials should be used, in standard dimensions, so that replacements could be constructed or the original could be repaired easily when no factory-made replacement is available.
The mechanical parts of switches should be individually inserted somehow from the top so that they could be replaced without soldering. Hall-effect or capacitative switches where the only moving parts are non-complicated parts, such as a plunger and spring. Perhaps where you move a magnet through a hole in the circuit board without it touching anything. Perhaps capacitative buckling springs with hammer in
individual housings.
Actually, you can type on the PCB of a capacitative keyboard directly just using your fingers, but that does not give you any tactile feedback whatsoever. It is very easy to press the wrong key, or two keys at once.
The controller should be on a separate board from the main board. All interconnects should be through standardized pluggable connectors, preferably with zero insertion force. The keyboard should be wired (to avoid needing a battery compartment) with a plug mounted to the chassis.
Case and/or chassis made of stainless or blued steel, and/or in anodized aluminium -- if they could be made thick enough. Keys made of anodized aluminium. Stainless steel allen-bolts, all of them in the same width and length, most likely M3 which is available everywhere. Stabilizers in hardened stainless steel.
Electric circuits are sealed from the top so that nothing spilled in-between the keys could cause a short-circuit. Top parts should be washable, designed so that there are no cavities that could retain dirt or liquids.
No painted surfaces. No plastic that could be vulnerable to solvents. No rubber of a type that would decompose in time.
I would advice against Liquid Metal. The big win with it would be that it can more easily be injection-molded into complex shapes, just like plastic. But it is too exotic, and therefore use of it would go against the serviceability criterion. Also, parts made of Liquid Metal are very springy and would make a very noise keyboard. In fact, the inventor of Liquid Metal has a demo object that he shows people: a bell made of the material, it has a stronger, more lasting tone than a bell would have made of any other material, except perhaps glass.
