Some nouns are already plural, such as series and data . In any case, this is a matter of morpohology alone. The possessive -'s attaches to the right edge of a syntactic structure which is often bigger than a word. The morphology and syntax of the two are completely different. Common nouns are nouns in our grammar.
Proper nouns are noun phrases.
Noun A noun is something that can be preceded by an adjective. This works provided that we exclude the proper nouns.