Sigourney Weaver's character has been revealed:
While a common guess was that Sigourney Weaver would be back for Avatar: The Way of Water to play the personification of the Tree of Souls, which Grace was brought to in the first movie in a failed attempt to transfer her soul to her Avatar body, in reality, she’s playing a Na’vi named Kiri, who’s the adopted teenaged daughter of Sam Worthington’s Jake Sully and Zoe Saldaña’s Neytiri.
https://www.msn.com/en-us/movies/news/sigourney-weaver-s-mysterious-avatar-2-character-has-finally-been-revealed-and-it-s-wild/ar-AAZ5Cuj?OCID=ansmsnnews11