Maybe the Prince Is -

[rowena_grunnionffitch]

No: HPFGUIDX 103807

   Merlin. He's been mentioned several times as a historical figure 
in the wizarding world and according to legend he was the son of a 
Welsh princess by an incubus, (for which read wizard) making him both 
a half-blood and a prince.

   Possibly in Book Six both the OOP and Voldy are after some 
heirloom or talisman left by Merlin? But my guess is the 'Half-Blood 
Prince' will be somebody entirely new - or somebody nobody's thought 
of. ;)