The missing Amendment is referred to as the "title of nobility" Amendment, but the second prohibition against "honour" (honor), may be more significant.
According to David Dodge, Tom Dunn, and Webster's Dictionary, the archaic definition of "honor" (as used when the 13th Amendment was ratified) meant anyone "obtaining or having an advantage or privilege over another". A contemporary example of an "honor" granted to only a few Americans is the privilege of being a judge: Lawyers can be judges and exercise the attendant privileges and powers; non-lawyers cannot.
By prohibiting "honors", the missing Amendment prohibits any advantage or privilege that would grant some citizens an unequal opportunity to achieve or exercise political power. Therefore, the second meaning (intent) of the 13th Amendment was to ensure political equality among all American citizens, by prohibiting anyone, even government officials, from claiming or exercising a special privilege or power (an "honor") over other citizens.
If this interpretation is correct, "honor" would be the key concept in the 13th Amendment. Why? Because, while "titles of nobility" may no longer apply in today's political system, the concept of "honor" remains relevant. For example, anyone who had a specific "immunity" from lawsuits which were not afforded to all citizens, would be enjoying a separate privilege, an "honor", and would therefore forfeit his right to vote or hold public office. Think of the "immunities" from lawsuits that US judges, lawyers, politicians, and bureaucrats currently enjoy. As another example, think of all the "special interest" legislation US government passes: "special interests" are simply euphemisms for "special privileges" (honors).