>There are only 4 Gyms in the main story, with an additional 4 optional ones. >Gym Leaders teams changed depending on how many badges you had.
>Nemona and Arven exist, but not Penny.
>You still get Koraidon/Miraidon similarly at the beginning of the story.
>The evil team is based on a marching band (Team March).
>The Champion of the region has disappeared, however the commander of the Team March calls himself the acting Champion.
>Jacq is a central character to the story, he later reveals he's the 4th Gym Leader.
>After defeating Jacq, the Team March appear and almost defeat the protagonist, but Miraidon/Koiraidon finishes healing just in time and defeats the evil team.
>The legendary takes the protagonist, Nemona, Arven and Jacq.
>It takes them to a legendary island that people think was just a myth, the island looks like 10 millions in the past / future depending on the version.
>Many Koraidon/Miraidon also live in the island
>Jacq upgrades the Pokédex to include the new island, and you get to name the island.
>In the legendary island there are trials like in Alola (Nemona specifically mentions that it reminds her of the trials of Alola from a long time ago)
>From here on, the true open world begins. You can freely travel between the main island and the legendary island. You can choose to go back to complete the 4 remaining gyms in the main island or do the trials
Full proposal document from 2019 with more details:
archive.org/download/titan20190107/titan%E6%A7%8B%E6%83%B3%E7%AC%AC%E4%B8%80%E7%A8%BF20190107.xlsx 
