All we know is that there are the new Solgaleo and Lunala forms that have fused with Necrozma. We were also hinted at potential new Z-Moves for Lycanroc, Togedemaru and Mimikyu. Not only that, but within the Pokémon Direct, they stated that new Pokémon would be added to the game that were not in Sun and Moon. We do not know if they mean Pokémon from past generations that didn't get to make an appearance in Sun and Moon or if they are new Pokémon entirely. Also, UsUm is stated to have compatibility with the Virtual Console versions of Pokémon Gold and Silver.
If I were in your shoes, I would probably wait until Ultra Sun and Ultra Moon to buy the game, as you never know if there may be new Ultra Beasts, Exclusive Z-Moves, or just new Pokémon in general. There also might be brand new features added to UsUm, which may change how players, both casual and competitive, play the game. And, if walking with Pokémon in the overworld is a plausible feature that arrives in the sequel games, it would kind of defeat the purpose of getting Moon as walking with Pokémon was such a fan favorite.
Long answer short, wait till this November to buy Ultra Moon.