The best replacement for proper noun is common noun.

The best alternatives to proper noun are rated as following:
★★★★★  common noun
★★★  noun
★★  pronoun
★  proper name
  common
  improper noun
  proper ajectice
  common nouns

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Speaking about proper noun, one probably shouldn't forget about common noun.

Other search terms related to proper noun and their ratings are:
★★★★★ common noun
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proper noun
common noun
noun