My top choices are
Arthur or
Alice.
I could also see
Alfred, possibly
Henry or
Frederick for boys and
Eleanor or
Matilda for girls. Personally I'd like to see
Eleanor or
Matilda.
I don't see
Margaret as likely for a first name. For one thing it's very dated in the UK and
William and
Kate seem like the sort of people who would choose popular names. I also don't see why they would honour
Princess Margaret. I doubt
William would have known her that well for one thing.
I could also see
Helena,
Sophia or
Isabella for a girl. Not long to find out anyway.