The current look Square Enix has given of Kingdom Hearts 4 seems to be implying that the start of this new arc for the series will be looking back at older installments for a more familiar feel. However, as nice as the return of Kingdom Hearts 2's reaction commands is, there are other design aspects to the levels and Sora's platforming abilities specifically that could revive the concept from the original title.
Though it would be unconventional to consider the series a Metroidvania, Kingdom Hearts 4 could make use of a similar level design in order to keep progression from feeling like a straight line from level to level. Considering that this was the way that Sora's platforming and the worlds were designed in the original game, it wouldn't be a far stretch to incentivize players to revisit areas as they progress through the story.
Kingdom Hearts as a Metroidvania
While the later games in the Kingdom Hearts series take a much more linear approach to progression, the original game periodically gave Sora new movement abilities in order to allow him to reach areas that had previously been inaccessible. Oftentimes, this would lead to special weapons and other items, sometimes even unlocking some of the best summons in Kingdom Hearts. It's similar to the unconventional "keys" that can unlock progression in games in the Metroidvania genre, which commonly take the form of heightened or doubled jumps and grappling hooks in order to allow access to new areas.
Considering that the first game fell a little short of delivering these Metroidvania-type keys, it isn't much of a surprise that later games would abandon the concept entirely. However, given that Kingdom Hearts was originally pitched as an action platformer, in the vein of Mario 64, it is interesting to see how much the series has deviated from that original idea. So, taking the chance to give Quadratum's level design in KH4 areas that are accessed through newly acquired abilities rather than basic story progression could make this new realm more interesting to explore and return to as the game continues.
Kingdom Hearts 4 Returning to the Series' Roots
With the loss of Kingdom Heart's antagonist Xehanort and Sora finally being introduced to the mysterious Master of Masters, Kingdom Hearts 4 is looking to be a major inflection point in the ongoing narrative. This is a new start that could even be pulling in new players with a refreshed storyline, and should likewise take this as an opportunity to revitalize the game mechanics. As a result, this could be the best time for the Kingdom Hearts series to return to the concepts that haven't been visited since the first game.
Interestingly, the Metroidvania genre has also been a hot commodity in gaming recently, especially among indie games that have mostly been succeeding on the 2D side of these designs. That being said, the immediate success of the Metroid Prime Remaster shadow-drop also proves that players are indeed interested in these titles in the 3D realm. It wouldn't be too strange for a series like Kingdom Hearts which has already dabbled into deck-building mechanics, and a spin-off rhythm game, to jump onto the biggest trend in gaming outside of the "open-world" buzzword.
Even going as simple as allowing items to be restricted behind a special type of high jump or glide could go a long way towards making this movement tech feel more rewarding and stretch exploration beyond a single visit to each world. To take that concept a step further, having worlds wrap back around each other, where progress comes in a seemingly non-linear fashion, could also help elevate the design beyond that of the unique worlds in Kingdom Hearts 3. It's a much more complicated design but would be worth the wait if Square Enix decides to give Kingdom Hearts 4 more interconnected worlds.
Kingdom Hearts 4 is currently in development by Square Enix.