Dd is 5 and not quite reading yet, but her favourite movie is trolls and I'm struggling with ideas for world book day for something not overly girly as she is not a girly girl that wears dresses, she prefers to wear jeans. I'm sure there are some books with princess poppy in by now.
Otherwise I was going to go for Alice or Dorothy.
Fwiw her cousin is going as Iron Man, who is probably in many books but did not start there.