The main addition would be a ton of character-driven post-game quests in new chunks of map. Give one for each Gym Leader, each E4 member, each "rival", Alexa searching for Volcanion, AZ trying to find where Xerneas and Yveltal went to try and stop any of this from ever happening again, Dexio and Sina as a pair, Diantha meeting a Diancie and her life as a movie star, the same Looker Bureau quest for Xerosic, and a posthumous Lysandre quest with Sycamore where you learn where exactly it all went wrong for him. All of them would have their own dedicated areas with proper time given so every character can become fully fleshed out, with some overlap between the events of each quest if you do them in certain orders. It'd basically make up a whole second campaign. Then there'd be a new Battle Frontier too, with the Maison, two new facilities, two from Emerald, and two from Platinum.
The main story would be mostly the same but Lysandre uses both Xerneas and Yveltal to power the Ultimate Weapon, and there's this sick-ass cutscene, where you hold the Zygarde Cube high above your head with the four stooges behind you lined up like you're the Power Rangers or some shit, and all one hundred Zygarde cells gather from across the region to your location in Terminus Cave to gather and form Complete Zygarde. Then the five of you ride on its shoulder as it heads to Geosenge to smash up the Ultimate Weapon, only to reveal when you get there that Team Flare upgraded it with a giant robot mode. There, you fight it in a special boss fight, where you switch mid-fight between the direct fight between Zygarde and the UW, and smaller fights where each of the five go off with their personal teams to fight off admins and grunts defending stones and terminals powering the UW so you can free the Pokemon hooked up to them and ultimately weaken the UW-bot so Zygarde can finish the job. Core Destroyer would automatically be used after you get it down to 0HP.
It would be wonderfully stupid.
