As it turns out, I am in a position where I can reveal some stuff about the anime. Technically nothing is set in stone, but it seems a surprising amount is planned out.

The current season will end with Clemont and Bonnie staying in Kalos and Ash and Serena traveling back to Kanto. There, Ash meets Gary again, who challenges him to a battle saying if Ash can beat him, he'll take him along to a trip to Alola for his research. Ash wins but Serena decides to stay and go on her own journey (she's planned to appear at some point in the SM series). Ash bids goodbye to all his pokemon save Pikachu once again, but it focuses extra on Greninja who Ash promises to "see again soon".

The first episode of the SM series actually focuses on Ash's male travel companion, based on the SM protagonist. Just like Ash, he sleeps in and almost misses getting a Pokemon. The whole first episode is basically him trying to get to Kukui's lab while shit keeps happening around him delaying his arrival. The end of the episode has Ash arriving in Alola.

Second episode focuses on Ash exploring the town when he lands, where he eventually learns of Kukui's lab nearby. Meanwhile we learn Team Rocket snuck aboard the boat and are also in Alola, and also plan to go to Kukui's lab. Ash arrives there at the same time "protag" does and they quickly become friends. Mallow is already there, and Team Rocket strikes. Ash defeats them, and "protag" and Mallow are impressed enough to want to travel with him. Kukui offers each of them a Pokemon. Ash takes Litten, "protag" chooses Popplio and Mallow is left with Rowlet, which Kukui hints she won't be upset with.