Lumiose City as a whole does feel integral to XY, if we count the entire city as the centerpiece of the region. We meet Sycamore, Lysandre, and Diantha there for the first time, Prism Tower is the Gym in the mid-game, and it has Lysandre Labs for the Team Flare climax and in the post game the Looker episode takes place there. So as a whole the city surrounding Prism Tower is a big centerpiece of the game, and one we repeatedly go back to, even if Prism Tower itself doesn't feel that significant (and frankly it was hard to be since it's a man-made tall but thin tower).
That said with that in mind both it and Mt. Coronet in DP really shared the same "role" in their respective regions which was being a centerpiece of the region and one that connected everywhere we went. Much like how Lumiose City, built around Prism Tower, connects to five different routes and all the route networks loop back to it, Mt. Coronet was a connecting piece of the entire region that we would visit several times along the way, and unlock more of it via HMs as we progressed through the game, and the route networks all connected via Mt. Coronet.
So it's kinda cool that the Legends games based off them really emphasize the fact that the two regions have that sort of "centerpiece" landmark that is the linchpin of the region, and Z-A is based entirely in Lumiose so Prism Tower, the city's centerpiece, is highlighted this time around and presumably some major story elements will occur there this time around.
(That said, I also do find it cool how the two regions are similar in that regard but also contrasting. One having a giant, natural mountain that stands tall at the top of the region, the other having a tall, man-made tower that stands tall in the center of what is the biggest city we've seen so far).