UPDATE 7/11/2019 –
Millie Bobby Brown denied she is cast in any Marvel movie during an Instagram Q&A.
Over the last few months there have been a lot of rumors and fan-casts of Millie Bobby Brown in The Eternals, mostly based on a set photo where she was visiting the Russo Brothers during the filming of Avengers. A couple of weeks ago, Kevin Feige told Comicbook.com that they didn’t have a role for her currently, and back in May she denied any involvement with The Eternals as well.
Today, Variety ran a report about Marvel’s San Diego Comic Con plans for next week; and they claim Millie Bobby Brown is part of The Eternals cast:
If this is true despite the two earlier denials, most people expect her to play the “Piper” character in the movie, which would be the MCU’s version of the Eternal known as Sprite in the comics.