I think William said Diana would be used if it was a girl in an interview ages ago. Probably as a middle name.
For a girl it's likely to be Elizabeth,Victoria,Alexandra,Charlotte. Lovely though Isobel and Olivia are,they're not exactly traditionally Royal names are they. The only reason the Queen has granddaughters called Autunn and Savannah is because the father is untitled and thoroughly disinterested in being Royal per se. Princess Anne's son,whose name evades me.
For a boy George,Arthur, Albert and um...all the others are in use currently aren't they?