Many of the in-game events have alternatives (such as Stufful replacing Snubbull). Which one you will have is randomly decided by the game, after your battle with Victoria here. Additionally, the guy at the main desk will hint where to go if you forgot.
The Grand Hall is a massive rest center in Reborn City, containing a Pokémon center, a PokéMart, and the registration for the Reborn league. It is here that you will choose a starter Pokémon, as well as have your first battles with your rivals Victoria and Cain.
In the back of the Grand Hall, the middle employee can inform the player the next location to progress to if the player has forgotten. There are also Trainers that get stronger as player progresses further into the game, that can be instantly rematched as a fast way to level up.
After you bought every pokemon in 7th street, and completed Titania's gym. Arclight will appear at the bottom right side in the Grand Hall. talk to him to initiate the Mega-Z Ring Sidequest. which will involved returning the stolen Pokemon UNEVOLVED.
if you have already evolved or traded them. you can trade with someone to get that same Pokemon. Or revert to a previous save and trade back and forth until its in you main save file.
Once you completed a restoration project in the grand hall. you gain access to the top and bottom floors. In the bottom floor, there is a room labeled "Pokemon starter storage". the woman inside will ask you to collect 1 egg from each starter Pokemon on her list on her desk.