What are the earliest sources that support or contradict the view of Sfas Emes and Chiddushei HaRim that each person (or Jew) has a unique mission in the world? Specifically looking for sources pre-Gra.
I don't have any pre-Gra sources for you, but R' Chaim Volozhin makes the point fairly explicitly in Ruach Chaim on the words of mishna 2:8 אם למדת תורה הרבה, explaining the words כי לכך נוצרת:
ואמר "כי לכך נוצרת" כי הנה כל אדם נברא לתקן מה, זה דבר זה וזה דבר אחר וכו
And it says "for this purpose you were created" as each person is created to fulfill something, this person one thing and this person a different thing...