Also,with Delta Stream, weather effects last indefinitely as long as Mega Rayquaza remains in battle. When it is swapped out for another Pokémon on your team, the weather immediately returns to normal.
Additionally, we assume that Sycamore’s theory is correct that the energy given off the first time the Ultimate Weapon was used was what triggered the creation of the Mega Stones (which, yes, does allow them to exist in Hoenn) and the story reason given for Mega Evolution basically boils down to bonds between trainer and Pokemon. So, outside of Mega Rayquaza’s unique method of Mega Evolution, what “secrets” to Mega Evolution is Rayquaza supposed to hold, exactly? If Sycamore’s theory is wrong, then why are there only records in Kalos of Mega Evolution? Why are the Mega Stones tied in some fashion to the Anistar Sundial?
There’s also the fact that Sycamore tells you that there are no records of Mega Evolution existing outside of Kalos as you go through X/Y. (Plus, it’s said that Lucario was the first Mega Evolution (hence the murals in the Tower), which, while that could be a matter of syntax given Rayquaza’s different method of Mega Evolution, seems to contradict the fact that legends and lore of Mega Rayquaza exist.)
There's been a lengthy Niconico broadcast of today in Japan, and as was promised there's been a major reveal to excite fans. Mega Rayquaza has been announced, and looks set to play an important role in the story.