Streep would play Topsy, the cousin of Poppins, who will be played by Blunt. Topsy, didn't appear in the beloved 1964 film starring Julie Andrews. The part would require the actress to sing, according to Variety.
Mary Poppins Returns is directed by Into the Woods helmer Rob Marshall. The movie will see Poppins, a magical nanny, reunite with a grown-up Jane and Michael Banks in Depression-era London.
"The iconic original film means so much to me personally, and I look forward to creating an original movie musical," Marshall said in a press release in May.
Mary Poppins and its sequel are based on the P.L. Travers book series of the same name. Blunt is succeeding Julie Andrews as Poppins, while Hamilton star Lin-Manuel Miranda will play a character similar to Dick Van Dyke's Bert.
Streep starred and sang as the witch in Into the Woods, earning a Best Supporting Actress nomination at the 2015 Academy Awards. The actress will next appear in a Florence Foster Jenkins biopic.