I think Bill and Bev (and maybe Richie) have the most complete arcs in the book. If we get any name actors, it'll be for one of those three. Adult Bev in particular has someone in her life who is an essential driver of the main plot of Part 2 so it might be the juiciest role.
I can definitely see Chastain boarding Part 2 of this - she's worked with the director before and is very fond of him, is supportive of this project in particular, isn't below signing on for franchise flicks (X-Men) the way say, an Oscar-bait actress like Amy Adams would be.