A few years ago, it might not have been a question whether or not Final Fantasy characters would make an appearance in a numbered Kingdom Hearts title. However, with the release of Kingdom Hearts 3 has proven that Square Enix isn't afraid to leave the Final Fantasy aspect of the series behind for future installments.
That being said, the initial backlash from having omitted the Final Fantasy characters from Kingdom Hearts 3 eventually led to them being reintroduced during the Re:Mind DLC. So, while there has been a pattern of leaving these fan-favorite characters out of the last few games, Square Enix may have learned a lesson about their inclusion in the wake of the most recent title.
The Last Kingdom Hearts to Prominently Feature Final Fantasy Characters
One thing that likely caught Square Enix off guard when fans voiced criticisms that the Final Fantasy characters had been omitted in Kingdom Hearts 3 is the fact that these characters had been gone for a while. Depending on how players see all of the extended Kingdom Hearts spin-offs and Final Mix rereleases, the last game in the series to feature anyone from Final Fantasy was Birth By Sleep. Even in this case, it was a short cameo of Zack at Olympus Coliseum that constituted the entire representation of the franchise throughout the entire game.
Since then, Dream Drop Distance and the various versions of Union X never included any characters that were specifically from the Final Fantasy series. The characters from The World Ends With You act as the extended canon of Square Enix's original series that appeared in Dream Drop Distance, but no major characters from the flagship franchise. Looking at the situation from the perspective of how long it had been since these characters had appeared, it makes somewhat makes sense that Kingdom Hearts 3 would be fine with dropping these extras altogether.
Re:Mind Reintroducing Final Fantasy
Well, as much as Square Enix thought that previous games had primed players to not see Final Fantasy players appear, the lack of these characters is still a major criticism to the story of Kingdom Hearts 3. As a result, a selection of these characters were later added to the game through the Re:Mind DLC that acts as the Final Mix upgrade for the first title to release in the era of online add-ons. It was only a handful of returning members that had found a home in Radiant Garden in Kingdom Hearts 2, but they were at least there this time.
This reintroduction garnered its own backlash, with fans claiming that the short cutscene and limited available dialogue was too little too late. As understandable as it had been that Kingdom Hearts had moved on from including Final Fantasy characters based on previous titles, they had still been a major draw for fans of the Square Enix side of the series. Additionally, there are a few ongoing storylines going on in this section of the series' lore, with the final fight between Cloud and Sephiroth having been teased as far back as Kingdom Hearts 2.
Kingdom Hearts 4
It would be difficult to imagine that these characters would be left out of Kingdom Hearts 4, especially given the hints of Final Fantasy Versus 13's Yozora connection. Combining what the series has been setting up in the last few releases with the backlash of leaving the characters out of Kingdom Hearts 3, and Square Enix is essentially guaranteed to keep them in the story moving forward. Fans might still have to wait for some time before the games get back to anything involving Cloud and Sephiroth, but Leon, Yuffie, and Aerith will certainly be returning.
There might be some theories to imply that the newly introduced World of Fiction could involve Final Fantasy, but much of what is planned there is still up to speculation. So, it's entirely possible that Square Enix will make a complete reversal of Kingdom Hearts 3 and make the Final Fantasy characters into an integral part of the story. Seeing that these combining franchises are still one of the biggest draws to the series, players can only hope for now that Kingdom Hearts 4 is able to make up for the previous title.
Kingdom Hearts 3 is available now for PC, PS4, and Xbox One.