I think it's going to be called Opal or Girasol. Giratina is the 3rd legendary dragon and represents death, so being the 3rd legendary dragon, it should be on the next game cover. Look at the eyes. it's the same as Dialga's and Palkia's! And all three legendary dragons come from a different dimension(Giratina comes from a world on the reverse side of ours). Plus look at its wings; Giratina's name is created from Girasol, a type of opal gem, the type on its wings, right? Diamond, Pearl, and Opal are all gems and Dialga, Palkia, and Giratina are the 3 legendary dragons representing time, space, and death. So the next one has to be called Opal or Girasol.
although it could be Arceus cuz all three of them can be found at the Spear Pillar. but it also couldn't be cuz Arceus wasn't even revealed yet. we shouldn't even know about it.
my best guess is Giratina with the name Pokémon Opal Version or Girasol Version. if it isn't, i'm going to hit my head on my comp when it comes out. :P