Isabela Merced has been cast in Madame Web and possible role based on a "leaked" plot summary is Anya Corazon. According to the leaked details, Anya is "an Avatar for the Web of Life" who is being hunted by a dark force across the Spider-Verse and Julia Carpenter (Madame Web) must keep her safe.
In Marvel Comics, Anya Corazon is Araña, a young superhero powered by a mystical spider-tattoo. After her mystic powers were taken from her, she received spider-like powers from a mysterious virus based on Spider-Man's DNA and was given the costume worn by Julia Carpenter during her time as Arachne. Anya became Spider-Girl for a while before returning to her Araña persona (with a new costume).
Anya Corazon was a pivotal player in the story arc where Cassandra Webb made Julia Carpenter the new Madame Web, and her current storyline has Araña traveling the Spider-Verse so her inclusion in the film seems like a very logical choice.