There once was a
Queen Emma(in the 11th century), so if I were in
Kate's position I would want to pick something that people hadn't speculated about. I think
Emma is pretty, modern (as in doesn't sound like a medieval name) and it is royal.
Emma Elizabeth Diana RoseEmma Diana Elizabeth Rose
and for a boy (thinking of something not used in the royal family for a while)
Alfred.
Alfred William Philip MichealAlfred William Charles Micheal
However, I'm guessing the most likely name will be
Victoria for a girl.
Not so easy guessing most likely for a boy but hope its not
George!!