New Pokémon, Alolan Forms or Mega Evolutions: The game could add up to 20 (a number dropped by Laura Dale, gaming journalist) brand-new Pokémon or forms/evolutions to Alola. Whether or not these will be compatible with Pokémon Sun & Moon is currently unknown.
Improved Graphics: GameFreak didn’t skimp in Sun & Moon, but their successor will apparently utilize the Switch’s improved capabilities to breath HD life into Alola and its inhabitants.
Compatible with Sun & Moon: While the new Pokémon might not make it to your copies of Pokémon Sun or Moon, there will be ways to transfer existing Pokémon to Stars through an improved Pokémon Box app. Many are hoping that Pokémon caught in the console version will also be able to be transferred to the 3DS games as well.
Switch-Exclusive Features: Pokémon Stars will not only bring improved graphics to what we experienced in Sun & Moon, but will also introduce new features not found in the portable games. While not mentioned in the rumors, the animations found for walking Pokémon do hint that a companion system will be coming back to the overworld similar of that in Pokémon Yellow or HeartGold & SoulSilver.