Mega Groudon will be Fire/Ground. Base Groudon will be remade Fire. Its ability will be like Drought but infinite duration
Mega Kyogre will become Water/Fairy and also have infinite rain
Mega Rayquaza will stop weather and not just be immune
Mega Deoxys will have best stats of all formes
Steven is the champion and has mega Metagross
Story is basically the same but you have to travel Johto to get mega stones