Underground tunnels. I know it's been done before in Goldenrod city and a couple in Kanto, but I'm talking like those, but expanded.
I live in Houston, TX, and a little-known fact is that there's a series of underground tunnels connecting everything in downtown together. You can get from the bus station to the museum to the high-rise office buildings by just walking underneath the city, and there are even coffee shops, restaurants, and convenience stores down there. I wanna see a place in the Pokemon world that mimics that atmosphere. The Goldenrod underground from GSCHGSS was a good start, and the Under from Colosseum was a step in the right direction. I want to see essentially a huge underground city connecting several cities, perhaps as an alternative to going over a water route or desert above land.