You can find an alley on 7th Street where scientists are working on a project. After defeating one of them, you may choose to help them. They will require collecting specific Pokemon for their new creation. After you finish the entire quest, you will battle a level 50 Type: Null, which you can capture. Before the battle, the scientist is killed by the Type Null since it still had the memories of its constituent Pokémon — and the presumably painful experiments it suffered.
Type: Null generally uses Normal-type attacks, so a Ghost-type would be immune to them while battling. This is especially useful for Type: Null's Take Down, which inflicts recoil damage on itself.
Carvana: Can be found in Lapis Ward (Post Restoration, Good Rod) or by using the Good Rod at the Aqua Gang hideout. You also get given a Carvana if you join the Aqua Gang.
Unfezant: Can be found in Ionia Valley (with high-leveled Pokémon, it's easy to evolve to Unfezant quickly)
Luxray: A Shinx can be obtained in the Onyx Arcade for 5,000 coins, which can then be leveled up to evolve into Luxray.
Breed another Shinx before trading in your Luxray, to save the trouble of obtaining another 5,000 coins.
After you gain fly by defeating Ciel, you can return to 7th street and the gate blocking the stairs behind the recovery machine will be open. It will lead you back to the Water Treatment center. It will lead you in the room Taka locked after you decided to battle/not battle him. In the said room is located TM58 Sky Drop and provides a shortcut back.
To the right side of 7 street, there's a dude wanting one of those "Rare candies". He will trade you a Piplup (With moves Icy Wind and Agility) if you give him one. You can find this in the doorway Bennett leaves open behind the mining rock. You will need Blast Powder in order to access it.
After escaping Subseven Sanctum, head to the building southwest of 7th Street. If you have the Helix Fossil in your bag, there will be a priest who will trade you a Sail Fossil (Amaura) or a Jaw Fossil (Tyrunt) in exchange for a Pidgeot. You can get the other fossil if you give him a Farfetch. You can get more fossils if you give him the Helix Fossil. If you have the Dome Fossil in your bag, they will not interact with you, so make sure you put the Dome fossil in your pc.
After the city restoration, Maxwell or Archer will appear asking the player to find jobs for the members of the gang as they are not need anymore due to the reconstruction. The members of the gang wants a specific job to work at, so you need to find the right job for them. The job applications are found around Reborn by interacting in the places they are in. You need to use the application in your bag next to the member who wants the job for them to accept it, since they will not ask for it. The list of jobs for the members are:
If you joined the Magma gang:
Darm - Circus
Nihil - Nightclub
Kriz - Spyce
Break - Medicine
Maxwell - Solice
If you joined the Aqua Gang:
Razzy - Sweet Co.
Defacto - Library
Pierce - Apophyll
Mannie - Construction
Archer - Silph Co.
After giving all the members a job, Maxwell will give you an Oshawott with a Choice Band while Archer will give you a Litten with Choice Specs.