The Enemies
The prime antagonist pulling the strings throughout the Kingdom Hearts series is Master Xehanort, also known as The Dark Seeker. We have seen several of his forms over the various games, and Nomura plans to finally finish The Dark Seeker Saga in Kingdom Hearts III, which encapsulates every Kingdom Hearts game released thus far. You can check the Kingdom Hearts Wikia for detailed information about Xehanort, but suffice to say many versions of himself should prove as foils to Sora and company for this finale.
Big bad boss man aside, Kingdom Hearts III looks to bring back the original monsters introduced in the first game: Heartless. We’ve seen Nobodies, Unversed, Dream Eaters, and Bugs take the place of Heartless in various other titles, but so far the only basic enemies to be shown in trailers are Heartless. Some are well known, such as Shadows, Soldiers, and Large Bodies, while others are completely new, like those shown in Olympus and the Kingdom of Corona.
Other assorted boss fights have been teased, such as the battle against the Rock Titan from Hercules and Baymax vs. Baymax in San Fransokyo. The Kingdom Hearts series is known for presenting boss battles against well-known antagonists from Disney films such as The Little Mermaid‘s Ursula and Peter Pan‘s Captain Hook, so it’s likely we’ll be facing off against the Stabbington Brothers or Mother Gothel from Tangled as well.