If Prince William marries before Charles ascends the throne he’ll likely be created a duke. Is there a title traditionally given to the eldest son of the Prince of Wales, like the king’s second son being made Duke of York?
According to Wikipedia, other titles used by males in the Royal Family include:
Duke of Clarence
Duke of Kent
Duke of Gloucester
Duke of Cambridge
Duke of Sussex
Duke of Albany
Duke of Connaugnt
Duke of Edinburgh
Duke of Cumberland
I don’t know if there’s any specific rules as to who gets which, especially as any of them may still be used at any given time, but as far as I know, the last time there was a similar situation, Prince Albert Victor was made Duke of Clarence, and the title is currently usused.