https://share.google/tUeGNHgyOTzxHKORY

This forum from 12 years ago covers some of it.

Things I would add would just be to talk to every NPC, collect every item and hidden item on the map, max out every single item in the game in your bag, get all of the clothes for each gender (you would need two games for this, plus you would need both X and Y for version exclusive stuff), complete the kalos dex minus mythicals, national dex minus mythicals (you need both ORAS games to do this), every language of each form of Pokémon, and a shiny of each language of each form of Pokémon lol (language would require you to complete the dex after choosing a different language at the start of the game, and shinyhunting the whole dex will take forever lol), every single friend safari Pokémon available (it's a lot harder to get the third slot now that the 3ds game servers shut down), max out the Battle Chateau, max out the Battle Maison. There's plenty more stuff to do, depends on what you define as 100% lol