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I'll give you 3 things:
Games are a technical sequel, and not a sequel: Game takes place a year later. It's the same protagonist but you didn't set out right away as you were helping your mom unpack and get Acclimated. In this time Hau and Lillie have defeated Team Skull and Aether with the help of Gladion and the Pokémon League with Gyms have been established. At the start of the game both Lillie and Hau are in Kanto.
2. Gyms and Trials coexist, Trials are roadblocks more or less.
3. Games is more difficult though only slightly. Heard they might be thinking about challenge mode but weren't sure how to make it work.