I think William and Kate will choose more or less traditional royal name for their first-born and future heir, taking in consideration historical background and close relatives who they want to honour.
That is my guess of male and female first (main) names.
Highly Possible Male Names :
George, Edward, James, Arthur, Philip, Alexander, Richard, David
Less Possible Male Names:
Andrew, Albert, Frederick, Henry, John, Victor, Augustus, Stephen, Edgar, Harald, Edmund, Alfred, Robert, Malcolm, Basil, Duncan, Constantin, Jeoffrey, Thomas, Gabriel
Highly Possible Female Names:
Victoria, Anne, Mary, Alexandra, Margaret, Madeleine ( Maud), Alice, Adelaide, Charlotte, Matilda, Elizabeth
Less Possible Female Names:
Diana, Frances, Helen, Beatrice, Eugenie, Louisa, Sophia, Augusta, Amelia, Emily, Caroline, Jane, Eleanor, Anabella, Arabella, Isabella, Joan, Agatha, Clementina, Emma, Cecilia
If it's a boy, it will certainly have one or some of the following names: George, Phillip, Charles, Michael.
If it's a girl, I have no doubts that one of middle names will be definitely Diana.