Big tin foil hat time.
So back in gen 6, people datamined Bank and found there were a third pair of games planned that were ultimately never utilized. This is obviously what was originally planned to be Zygarde's thing as the 2015 release, but most likely got cancelled due to GF wanting to get gen 7 out for the 20th anniversary. Zygarde infamously got shafted as a game-long Cell hunt, but there was also another interesting addition: Guzzlord.
Guzzlord is the final UB encountered in SM's postgame. You find it in the last room of
Resolution Cave, which is a replica of...
Terminus Cave's final room. It is the thematic antithesis of Zygarde - protecting the environment at all costs vs. mindlessly destroying it. Lastly,
James Turner himself alluded that there are intentional similarities between the two.
Fast-forward to 2024, and we have logos for a Kalos revisit named Z-A. The Z is a pattern of dark hexagons, while the A is green scales.
And now is where It All Comes Together(tm).
My crack theory is that the cancelled Kalos revisits were Pokemon A and Pokemon Z. Zygarde and whatever Guzzlord was initially conceived to be were the mascots, but the games were casualties of GF wanting a new gen for the anniversary. Zygarde was repurposed as the Cell hunt, and the other mascot was repurposed into the UB we now know as Guzzlord - the latter being placed at the end of Resolution Cave as a sort of development in-joke. Z-A is named that way to commemorate both cancelled titles and finally being able to explore their plotlines. With all of this, I imagine the overarching theme of PLZA will be taking the end of something (Z, the last letter of the alphabet) and using it to create something new (A, the first letter of the alphabet) along with themes of industry consuming the nature that was there to start with.
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