I think the one thing that makes me hesitant on this making $300M+ is that, when compared to Disney's recent fantasy films that made $300M+, there doesn't seem to be anywhere near as strong of a hook.
Alice in Wonderland came out when Johnny Depp was at his peak of popularity, it was the first time since 101 Dalmatians Disney brought one of their famous animated films with a live-action spin (I know it's not actually a remake of the animated film, but it sold itself like that), and arrived as the first major 3D event film since Avatar, which breathed new life into the format.
The Jungle Book sold itself on its groundbreaking visuals, which was hammered down people's throats, and the marketing heavily pushed the film as being something that needed to be seen in 3D and IMAX. It also had critical acclaim.
Beauty and the Beast was capitalizing on 90s nostalgia, which is extremely potent (kids who watched the animated movie have kids of their own, or want to recapture something they love), and it also had Emma Watson playing another Hermione-esque character, which gave it a lot of appeal from kids, adults, and teens.
Mary Poppins Returns has none of that. Emily Blunt isn't a big name like Depp, it won't have awards-worthy visuals, I'm not sure if it'll even have 3D, and while it's still popular with the kids, it doesn't have the same kind of 90s nostalgia power that something like Beast, Aladdin, or Lion King had or will have. It'll likely fit in somewhere around $250M, and I think we should be happy with that number.