Theory on the book 5 Advert:

Below is an advert somebody sent in to The Leaky Cauldron - it shows Harry as he is on the American cover as well as 3 additional people we haven't seen before. I think they may be from left to right: Ron, (either) Fudge or Lupin and Mad-Eye Moody. Though Mad-Eye is the only one I'm absolutely sure about. I think the person on the far left is Ron as he resembles Mary GrandPre's Ron in this picture:

Usually when GrandPre puts people on the front cover (and not a scene), it means they play very important parts in the book (for example in book 4, it was Fleur, Cedric and Krum).

Also notice the figure (circled) to the right of Harry's wand, it looks almost like a projection - though it can't be the Dark Mark as that is a skull, could it be the siluette on someone in a doorway? And could it be the same person in the window in cover art of the Deluxe Addition?

Hmm, ponder that. Have anything to add? Email me.