Referring to the earlier theory that meteor falls could hold the Diancie mega stone I completely agree. Hoenn has so many potential areas that could house legendary Pokemon, meteor falls is perfect for Jirachi and Deoxys however those will most likely be event pokemon so the it seems that the Diancie mega stone is a perfect fit for the meteor falls. Realistically there is no other place for it to be, as I believe it will be an item received after you beat the game it may be in the section Steven is in the final battle, I feel Granite Cave is too close to the start of the game and perhaps Altering Cave could house it (if it makes a return) but it doesn't seem likely as that place is made for a different purpose. Of course as people have mentioned it could be in an entirely new area, there are several islands shown on the new Hoenn map and of course there are several islands visible but not accessible in the original games so it could be anywhere!
