We mentioned this after banishing Primal Groudon/Primal Kyogre and Wallace will also point this out after you defeat him: some changes have occured to Hoenn following your epic battle with the primal beasts and it’s best if you spoke to Professor Birch back in Littleroot Town for the details.
According to the prof, a number of new wild Pokémon have been sighted, ones not normally native to Hoenn. If you go back to old locations, you can find these new Pokémon and add them to your ever-growing collection.
To make the additional cataloging easier, Professor Birch will upgrade your Pokédex to the National Pokédex , which can catalogue all 721 species available in the game. Most importantly, though, if you complete the National Pokédex you can acquire the Shiny Charm from him.
It’s not just new Pokémon either, as you can also collect Mega Stones from the non-Hoenn regions, such as the Mewtwonite X in southwest Littleroot! Well, what are you waiting for? See the relevant section for more details.