One thing that players immediately latched onto when Square Enix first revealed the cover art for Kingdom Hearts 3 was the inclusion of a strange little creature that many fans might not have even recognized. This was the addition of the little Darkling, that implied that Kingdom Hearts' Keyblade War would have a larger impact on what was being primed for the final game in the series than it ever had before.
Of course, Kingdom Hearts 3 released and that little Darkling on the cover art turned out to be little more than an Easter Egg for attentive players that had played Kingdom Hearts Union X for long enough to recognize it. After that, the Darkling was shelved and the main series of games haven't seen any mention of this special type of Heartless that has the deepest lore implications of any enemy in Kingdom Hearts.
The thing about Darklings, is that they aren't just any Heartless, going well beyond the specifics of Emblemed or Pureblood in how they tie into the established lore of the series. These are an enemy first introduced not long before the player character of Union X goes to Kingdom Hearts' legendary Keyblade Graveyard, and are revealed to be the Heartless of former Keyblade Wielders. So, aside from being a serious retcon for the series, Darklings have the potential to actually be characters that fans are now familiar with, and should work their way into becoming a permanent fixture in future titles.
Finally Connect Union X to the Main Story
There were a number of times in Kingdom Hearts 3, where it felt like the stories from the Union X era might finally connect to the main series, but most of the crossover took the form of cameos. Between Ephemer showing up for a few seconds and the hints that either Skuld or Ava might have wound up as a prisoner who got experimented on in Ansem's castle, the connecting threads were all there. However, those loose ends were never tied up, and instead left dangling while Kingdom Hearts heads into a new direction for the foreseeable future.
Then, beyond all of those red herrings, the story of Kingdom Hearts Union X has continued forward after the Keyblade War and into a whole new mystery about the Darkness. The many narratives are setting themselves up to come together, and the Darklings might actually be the best way to do it. In fact, using the Keyblade Wielders turned Heartless now could be the best way to solve for the time difference between Union X and the main series.
Make The Darkling Someone That Players Know
The best way to utilize the Darkling moving forward might be to have one of the characters that players are now familiar with be turned into a Heartless and force Sora to fight them. This is something that has already been done in a slight way with the fight against Aqua in Kingdom Hearts 3, and could be touched on again in a way that has less emotional weight for Sora than it does for fans of Union X. It could be a situation where the characters in-game don't know who it is, but the Darkling makes some sort of movement or sound that hints to the players that another Keyblade Wielder has been defeated.
This could also take the "recompleted" concept and turn it back towards the Heartless instead of just the Nobody's that were able to suddenly come back one game after being defeated. That type of setup and payoff in Kingdom Hearts 4 could wind up making way more sense for how the ancient past characters show up in modern times. Instead of relying on the divisive time travel rules that later games introduced to the series, some of these Union X characters had to go the long way while turned into a Darkling and are finally saved by Sora.
Kingdom Hearts 3 is available now for PS4 and Xbox One.