Since the discussion has run out of steam I thought I'd throw in my two cents. We can categorise headliners into three distinct groups:
Known knowns (Elton John) - confirmed, announced, set in stone
Known unknowns (Arctic Monkeys) - tour dates aligning, history with the festival, act at the peak of their powers (although maybe not in a live setting)
Unknown unknowns - Is there a curveball that hasn't been considered? What of the wildcard, the joker in the pack, the leftfield choice which has been kept so tightly under wraps we've not heard a peep?