No, I don't work at Game Freak, I'm not a Chinese Localization theme member, and even a reviewer. I'm just a guy who managed to get some "leaks" from the upcoming games.
Because of this, I don't know much, so please do not insult me if informations I'm gonna reveal would be pointless for you.
>There are indeed new forms, but they are just the cover Legendaries, who are in fact just the "spirit" of Solgaleo or Lunala with Necrozma's pieces attached. So yeah we can call this a new form...I suppose? Obviously stats, name, and typing are changed but I don't know details.
>No Battle Frontier, no new areas, just a few more trainers and a few more items. The difficulty is increased though.
>Two new cameos from the old games: Cheren and Clair. Clair is totally redesigned and now she seems more "draconic" than before. Cheren keeps his appearence from BW2.
>Of course the Dex is bigger, but just a bit. Only 70 mons are added, most of them are 4th and 2nd generation Pokémons.
>The plot is basically the same for the exception that Lusamine uses the flute to summon the legendary since you managed to defeat her making impossible to summon UltraBeasts. Then she uses its power to open a Wormhole. But soon Necrozma rush into the Foundation and absorbs the corrispective cover legendary, apparently "killing" nebby and using its ghost and its armor to create the new form. That's it. You defeat Necrozma, lusamine falls in coma, Nebby manages to get rid of Necrozma who flies in Ten carat Hill aaaand...the story remains the same. No Kanto. No trainer Lillie. No Mohn. Sorry to disappoint you.
>Poké Finder is updated with some new functions and new Pokémon to appear.

Feel free to ask, but I don't know that much.