On the assumption we're supposed to be pronouncing it "Z to A", would that mean we are returning to a beginning of some sort? The origins of modern Lumiose City? The popular use of Mega Evolution? The (re)discovery of Infinity Energy? Or maybe its plot revolves around fully evolved Pokemon returning to their beginnings?
The Fletchling in the final shot follows the arc that the Talonflame took in the shot before it. It was probably just visual flair, but I want to entertain the possibility that this is the first hint to the plot of the game. Infinity Energy's connection to evolution was a big theme of Gen 6, and now would be the perfect time to revisit it. Maybe part of the plot involves Pokemon evolution getting messed with somehow?
I'm imagining the problem starts with Pokemon's evolutionary states in flux. Some getting devolved like their energy is being taken away. Others getting engorged with energy, powering them up or causing them to evolve. This sudden change causes a spiked aggression, making Alphas more common. The equivalent to the Noble Pokemon end up mega evolving and becoming enraged due to not having a trainer with a keystone to balance them out.
If Z-A is set in the past once again, this could be when knowledge that mega evolution can be controlled with the help of a trainer is spread. Much like in Legends Arceus, people would be fearful and afraid of these rampant mega evolved Pokemon. The hopes of a city made for Pokemon and people coming to a halt until the protagonist comes in and calms the raging mons with the help of Korrina's ancestors. It would help reconcile the Alola dex entries and Mystery Dungeon with what was established in Gen 6 (which the Gen 6 anime kinda implied was already the case).
We know Zygarde is going to be involved in this, and the chaos following Pokemon's evolutionary states going in flux seems like the sort of ecological disaster it would be concerned about.
(This is assuming that they don't go a completely different direction instead of retreading similar ground obviously. Also, Zygarde getting drained and having to gather its cores would be a good way to have it with you from the beginning, but the Raidons already did that and this is getting waaay too long already.)
