Satoshi Tajiri could give you a special beta pokemon after you defeat him! This pokemon will be like Mew as such no one knew this pokemon existed, some sneaky employee added this just before it's release! The game will say it has 230 new pokemon when there is actually 231!
The gen 6 version of the 'Mew glitch', except it can be actually obtained through regular gameplay.
I think this would be the coolest thing ever!
It would also have awesome stats just like Mew!