Amy Adams/Lois Lane
Not only does Amy Adams literally look like the grown up version of the young girl, the 'lois lane' line from It1 (from the creepy slightly Christopher Reeve looking pharmacist) gets repeated in Chapter 2 and would've worked so well/meta especially with all the movie nods going on (The Thing, The Shining, Nightmare Elm St 5, the Stephen King cameo etc) whereas with Chastain it was meaningless
I guess the director previously having worked with Chastain clouded his judgement on casting