The French title to the sixth book,
Harry Potter and the Half-Blood Prince can possibly offer a new insight into who the Prince could be.
There have been debates as to whether Half-Blood Prince means that prince will be of half-blood or whether it means he will be the prince of half-bloods - sort of like the 'Prince of thieves' concept in Robin Hood.
Céline has let us know that the French title for the sixth Potter novel is
Le Prince au sang mêlé.
This directly translates into English as 'the prince who is a half-blood.'
Source:
Veritaserum