Not sure they'd go with the name Caroline, just because of how hated within the Royal Family Caroline of Brunswick was who was the wife of George IV. She was hated by the family but loved by ordinary folk (sounds just like Di!). George tired to divorce her and had her locked out of his coronation and she died a miserable death.
If its a girl I think it'll be something safe like Alice, Alexandra, Mary, Helena etc. There have been a few Royals called Maud in the past and the middle names of Victoria's daughters included Adelaide, Augusta, Louisa, Alberta so maybe if they wanted something different they could go for one of those?
If its a boy, I'd go for George/Albert after the Queen's dad or maybe Frederick.