My best guess - assuming this is actually a thing - is that this series will be either an interquel or companion to the next mainline game, focusing on one or multiple of our existing protagonist groups. There's a word for this - "hyperserial narrative" - which describes stories that are broad in expanse, consisting of multiple micro-stories which add up to a broader meta-story across multiple media. KH as a series has always toyed with this structure but has leaned into it more explicitly in recent years; hence the on-the-nose MCU-style branding with the 2020 trailer, and the way ReMind/MoM/the mobile releases have outlined a future in which multiple branching stories will intersect/interact.
The supposed scope of the next game - Riku/Yozora/Sora in Quadratum - is already potentially so vast that it would be incomprehensible to also manage progression on
all of the following narrative threads headed by our multiple protagonists in the same game: Mickey in Scala, Kairi/Aqua, Ven and his past, the Twilight gang (Subject X etc), and the potential return of Ephemer/et al, not to mention all the potential sub-plots (e.g., Laurium/Elrena, Foretellers, Xehanort's classmates/upperclassmen), which could be woven into any one or multiple of these narratives.
The scope here is huge, and it's hard for me to envision a world in which, with this ever-expanding "hyperserial" to work with, Nomura would opt to use a major medium such as a TV series for a reboot, instead of progressing one or two these essential protagonist threads. Sure, you could make the argument that KH has managed multiple storylines before through the release of multiple games, but I don't think that's really applicable here for a couple reasons:
- With rare exception, few of those games actually progressed chronologically simultaneous plotlines that were moving toward a common goal/outcome. At most, we had CoM/Days, and Days was made after that main arc was completed in KH2 to fill in some gaps. The same can be said of 0.2. Of course, some threads from Days later found their way into the broader meta-story that culminated in KH3, but that's a very different approach from what we're seeing now, with the stage/next steps clearly outlined for each group post-MoM.
- I think the days of smaller budget handhelds to substantially move the plot forward are behind us. The mobile games are obviously an exception, but it's important to note that these stories are not driving the present narrative/continuity forward; they provide lore and context, which can certainly be woven in later, but no one can in seriousness speculate that, for example, Lea/Isa/Subject X's story will be told in chibi-gacha format. These teams are working with hardcore hardware and major advances in game dev capabilities and it's clear Nomura has an ambitious plan for the future of the series.
The dev cycle for future games won't be short but there are a
lot of simultaneous narrative threads to push forward; it would be a smart move on Nomura's part to leverage the Disney+ series for this purpose, and I don't see how it would "over-complicate" things as some folks seem to imply. If anything, it could help streamline what right now appears to be a chaotic clump of multiple moving parts.