I don’t see why an American or Australian or any other actor from any other country couldn’t be cast.I'm of mixed mind about this. I've always thought that due to time constraints, the movies had to keep a LOT out, and thought (hypothetically) that a series style would be able to tell the entire story more effectively. I mean, if you just watched the movies, you wouldn't know how vital Neville is. You wouldn't know Voldemort's backstory, and so much more. There's also an opportunity for them to not completely butcher Ron and Ginny's characters.
However, even disregarding anyone's personal thoughts on JK Rowling, it will likely be hard for people to connect to the series, as the movie actors are so ingrained in peoples minds. Whoever plays Snape, for example, will have a lot to live up to. I mean,
even if you see the play Harry Potter and the Cursed Child, the actor who plays Snape, at least in the show I saw, mimics Alan Rickman's voice).
I do hope, though, that they do better casting (age-wise) for the adults. I mean, James and Lily were 21 when they died, but were played by 40s-ish actors. Snape should have been 31 at the start of the series, but Alan Rickman was 55 when the first movie was released. And I hope that they stick with an all-British cast instead of casting well-known Americans (I know a lot of people in the past have "fan cast" Adam Driver as Snape) and having them do British accents.
I always thought it would be fun, if they were to do a reboot, if we saw the series from someone else's POV. Like Seamus. Or Crabbe/Goyle. Or even more entertianingly, Luna.
A lot of fans have also said that rather than a retelling of the story we know, they'd like to see a prequel focusing on James, Lily, Sirius, Lupin, Snape, Peter, etc, and their time at Hogwarts and leading up to the events we know.
I haven’t read the books in a while but I don’t recall some of the characters country of origin being brought up.
The school is in England certainly, but I would imagine a Wizard school would actually logically attract teachers from around the globe like any specialty school.
And British actors do American shows consistently.
Whoever is right for the role or who would be an exciting choice who be who should be cast.
And if the actor can do a convincing British accent, let them be cast if they want an all “British accented cast “