I think it's another girl but either will be great.
For a boy, I'm leaning towards Philip. I definitely think Philip is gonna be somewhere in the name but I also could vividly imagine them using it as a first name. I don't think they'll use Charles – I think it's very obvious that they already honoured him with Charlotte – but I could imagine them honouring Kate's father since he wasn't included in George's name, so either Michael or Francis. I also think there's a good chance of either Arthur, Alfred or Albert being somewhere in the name. My initial guess would be: Arthur Philip Michael.
For a girl, I'm still set on Matilda. I think it's such a beautiful name that would go well with George and Charlotte. But at the same time I also feel like I'm just projecting my own preference over here
I actually think names like Alice or Rose are more their style. Or maybe Josephine? I could see them honouring Carole – either with her actual name or by using Caroline. And then I think there could be a chance of Philippa being one of the middles since it would honour both Philip and Pippa. My initial guess would be: Matilda Caroline Rose or Matilda Philippa Rose.