Well if they keep to tradition, then Grey will probably not explain what happened to N. The 3rd game usually just adds more non post-game storyline and new features. The remakes of them are what usually adds a lot more story in the post-game like in FRLG and HGSS (The Giovanni event). Grey will most likely add some sort of Battle Facility of some sort and a more in-depth story to Kyurem and the others. Then again, this is all speculation and they don't have to keep to tradition.