Considering that the Y button now launches the Union Room, it's likely that the GTS and Geonet will also be integrated there just like the PSS/Festival Plaza as a quality of life feature. Since the physical GTS location still exists, it's very possible that the building will be home to the Battle Frontier as they could easily add more warp tiles to the main floor to warp you to the Battle Park/Tower and each other battle facility in Johto. There is precedent for this as lorewise Jubilife City is the main communication hub of Sinnoh, hosting the TV station, the Poketch company, and the only school in the region. Wi-Fi is canonically invented here and the original DP featured a Wi-Fi Battle Room challenge and the Wi-Fi Club in Pt/HGSS allowed you to play multiplayer Battle Frontier. They could also allow people to unlock it before postgame in a faithful way, as Mystery Gift was originally unlocked via entering a secret code in Jubilife TV, but now it's a button on the main menu so something else will have to be unlocked from that NPC. They can't use "EVERYONE HAPPY WI-FI CONNECTION" again because technically it's not Wi-Fi anymore, but BATTLE FRONTIER would also fit for the last two words.