The covers reveal some clues

On the US cover, it looks like Harry is wearing the Slytherin locket, and that he's placed a medicine pouch over it to keep it concealed. I have a hunch that Harry will reveal to Voldemort (during the "final battle") that he has destroyed all the horcruxes. Either that, or Voldemort will destroy the locket himself, not realizing what it is because it's covered up.... The locket itself, once "disarmed" of it's "soul bit" might act as a protection amulet...one that actually works. Voldemort seems either repulsed or frightened by what he's watching, and he also looks like he is moving to keep Harry from whatever he's trying to catch. Very reminicent of the first cover showing Harry's pursuit of a golden snitch. Wonder what he's reaching for this time. It would be lovely to think that Fawkes is bringing him something, like in CoS, but not the hat. Maybe the sword is the object we cannot see? Maybe it's a wand he's reaching for?

On the UK children's cover, though Hermione and Ron are worried by whatever they see (whatever is guarding the treasure... dragon, perhaps?) Dobby (or perhaps Kreacher merely following direct orders) is on Harry's back with sword raised... to protect Harry. This is all so Harry can grab something that has caught his eyes. If you follow his stare and see what's reflected in his glasses, it appears to be the gold goblet that is sitting right behind the silver helmet with the dragon motif on it. It looks like he's trying to grab at this goblet. Could it be Hufflepuff's cup? Possibly. We can't see if it has a badger on it, since most of it is hidden from our view. This reminds me very strongly of Harry in the first TWT task, when he's trying to get the golden egg from the dragon's nest. This, and Hagrid's comment about the dragons-guarding-high-secuirty-vaults-at-Gringotts rumor, leads me to think they could be dealing with a dragon here. Which breed, who knows? But a dragon could also explain what look like burn marks on the faces and arms....

On the UK adult cover we have what can only be Slytherin's locket. Since the adult covers are more specific to the book title, I strongly believe the Deathly Hallows are founder heirlooms, even if they are not all horcruxes now. This would include the sword (not horcrux), the cup (apparently a horcrux), the locket (definitely a horcrux), and... maybe the wand that used to sit in Ollivander's shop window. Swords, Cups, Pentacles, Wands.... Sounds right to me. :)