Mega Evolution is a fairly recent concept in the Pokémon world. Since it’s so recent however, it will be a long time before it is included in Pokémon GO, if ever. Here is how Niantic could include Mega Evolution in Pokémon GO.
I believe that before Mega Evolution can be introduced, held items must be. This would not only give things like Leftovers a place in Pokémon GO, but it would make it possible to hold Mega Stones.
Key Stones are pretty simple no matter how you look at it. One option is to just have it in the inventory when the game is downloaded much like the Camera or Infinite Egg Incubator. You could also get one when you reach a certain level. I feel that it could also be ignored if the game allows you to Mega Evolve without a level requirement.
Introducing the Mega Stones themselves is pretty simple. You would just need to add the stones for the Pokémon already in the game (like Alakazam and Pinsir) so they could be equipped. If equipped, that Pokémon would evolve upon being sent into a gym, with only one Mega Evolution. Alternatively, Mega Stones in Pokémon GO could function as regular evolution stones and permanently Mega Evolve your Pokémon.
How do you think Mega Evolution should be introduced in Pokémon GO? Let us know in the comments below!