There are 2 competing textual varients in the Greek sources. The word for "good will" (eudokia) shows up in the nominative and the genitive. If it's genitive it modifies humankind if it's nominative it's parallel to the first half of the sentence meaning, in Heaven let there be glory to God, on Earth let there be good will to humankind.

That was fun! I read that verse earlier today and I didn't know about the version with genitive case.