A new Heartless introduced in Kingdom Hearts 3 stands out as a surprising boss fight that displays some attributes that aren't exactly common with the enemies found in the rest of the series. This would be the Lich, which is one of few intelligent Heartless in Kingdom Hearts that seems to be able to plan and act on more than just survival instinct and a hunger for hearts.
If the extended lore of the early games is to be believed, there are only two people in the history of Kingdom Hearts to retain their intelligence while Heartless. These would be Ansem Seeker of Darkness, aka the Heartless form of Xehanort in Terra's body, and Sora, the latter of which only had mild control as players march the Shadow through Hollow Bastion.
Seeing the Lich somehow able to act with intention stands out against many of the other enemies that are clearly stated to have lost their ability to think in earlier games. There are other beings of darkness in Kingdom Hearts that are still able to think, but many of them are either working with darkness like Xehanort or haven't completely turned while able to talk and think like Nightmare Chirithy. So, the next few games in the series might start introducing even more of these Heartless that begin to act of their own accord.
The Lich Capturing Hearts and Running Away
The biggest difference between the Lich and any other Heartless in Kingdom Hearts is the way that it doesn't consume hearts the same way others do, instead choosing to capture them and ferry them to the Dark Abyss. This alone at least shows a slight level of conscious thought, as the Lich doesn't follow the same instincts as the standard enemies in the rest of the series. However, the nature of the fight against the Lich also implies that it is able to think clearly enough to attempt to stay alive, even though it is ultimately defeated.
Throughout the fight, the Lich continually runs away and attempts to escape with the hearts that it has captured in order to drag them into darkness. Few other Heartless Sora has fought in Kingdom Hearts have run away from him, with the one's that did being limited to gimmick enemies that disappear if the player fails their puzzles. Everything else will fight Sora until it dies, giving into the instinct to capture and consume hearts so deeply that they don't have much of a sense of self preservation.
The Darkness with Intelligent Heartless
Many of the mysteries currently appearing in Kingdom Hearts center around a new enemy that has recently appeared in Union X, who is known as simply The Darkness. From what has been released so far, this new enemy seems to be a sort of mirror version of the heroes, but comprised of dark instead of light. However, if this Darkness is going to be a threat moving forward, then it may need some companions to help it accomplish its goals, especially with how much Kingdom Hearts loves its evil organizations.
So, the Lich could be the introduction to this type of enemy, a new breed of intelligent heartless that works together with The Darkness. At the moment, much of this storyline is still stuck in the Union X's ancient past storyline, but there are hints that the two might be converging soon. This still leaves a lot of questions in the air for Kingdom Hearts 4 to try to answer, but this might be the closest look players have gotten to what the new threat in the future of the series is going to look like.
Kingdom Hearts 3 is available now for PS4 and Xbox One.