Fans can't even decide WHEN this game is supposed to take place.
In the future? Because of the "holograms" and the city already being built and now getting "re-developed"?
In the past? To reflect when the Eiffel Tower was getting built, and a lot of Paris was torn down to redesign the city. With the "holograms" just being a way to represent old school blueprints in 3D?
In a alternate present? Or maybe some time travel stuff? Or something like Assassin's Creed, where you re-enact the past with VR technology in the future?
If the game really does take place entirely within a already built Lumiose, that's actually a really big change. I'd guess that those 5 circles, which used to be plazas, will be full of stuff and probably expanded so that we can catch Pokemon in them. If it takes place in the future, maybe there's even something like the Terrarium in Blueberry Academy below the city.
And, personally, I do hope they've tweaked the combat a little. Everything with the catching mechanics was great in Legends Arceus, but something about the battles themselves always felt a bit off. Maybe there just weren't enough of them to really get used to the new mechanics, but something just felt unfamiliar with the battles.
Maybe our character is basically hired as a "dog catcher" for the city. And we just catch the unwanted Pokemon that are springing up different places in outbreaks around the city. That feels almost too dark for Pokemon, but maybe?
I've even see some people speculate that this Legends game won't play anything like Arceus did. Much like how LA experimented with changing the catching mechanic and a open world, THIS game might experiment with making Pokemon battles real-time. This might be the feature that the Synchro Machine was teasing. We might end up controlling our Pokemon during battles, with more action-oriented combat. We do see most of the trailer from the Pokemon's POV, after all.
It kind of depends. Does Gamefreak see "Legends" as a particular style of game, or do they see this sub-franchise as just a excuse to test out new mechanics that could be brought into the main games?
It's going to be interesting to see how accurate that trailer really was later.