Okay, so as someone who’s taken quite a bit of issue with how they handled these two in Season Three, here’s how I’d like to handle their arcs in that season:

For Alya, I’d want to focus more on her civilian life and specifically her journalism career, using her character to highlight a very real issue in modern journalism, that being the focus on sensationalism over accuracy. With Lila’s stories about being Ladybug’s best friend - and how Alya contributed to bolstering that lie by interviewing her for the LadyBlog - I would’ve loved to see an Alya who struggles with the temptation to focus on delivering ‘big stories’ based on Lila’s testimony, even if she questions it’s legitimacy - or publicly denounce her at the risk of not attracting the attention necessary to make her first steps into pursuing what she loves. Her friendship with Marinette is also underlined by this - while she initially maintains the theory that Marinette’s just being jealous over Lila’s interactions with Adrien, a conversation they’d have near the end of Chameleon would weigh heavily in the back of her mind, as a small part of her wonders if Marinette might be right. As Marinette grows closer to Luka and bonds with Kagami despite the latter entering a relationship with Adrien, Alya’s conviction in her initial viewpoint continues to weaken, until she learns about Lila’s role in causing Kagami to become Oni-Chan and completely rejects her, committing herself to the truth and maintaining her integrity, both as a person and as a journalist. In the end, Alya would play a crucial role in exposing Lila when she tries to frame Marinette for serious crimes and forcing Lila to flee (eventually taking up the new identity of Cerise Bianca), with Alya affirming herself as someone who prioritises the truth, even as a hero with illusionary powers. 

For Nino, I’d focus more on his role as Adrien’s best friend and close confidant, especially after Adrien (not Marinette) gives him the Turtle Miraculous to become Tortue Verte (yes, I don’t like the name Carapace). As Nino proves he can be trusted with the responsibilities and secrets that come with being a hero (i.e. him and Alya sharing their identities to one another is accidental), Master Fu decides to take Nino under his wing and begin training him as the new Guardian, having realised that Marinette and Adrien are under enough pressure in their own lives and, as the wielder of the Turtle Miraculous that prevents tracking, Nino is the most reasonable choice to protect the Miracle Box. Nino’s training with Fu ultimately comes to a head once Master Fu either dies or loses his memories (the temporary heroes’ identities aren’t compromised here, since Lila becomes Miracle Queen instead of Chloé), with Nino fully taking over as the new Guardian and deciding to have a mutual identity reveal with Marinette, Adrien, Alya and a properly redeemed Chloé, with Marinette creating charms that ensure any Miraculous-related knowledge will be hidden from Hawkmoth/Shadowmoth when he tries to akumatise them. This leads into season 4, where Marinette and Nino find themselves at odds over their contrasting views on how to handle the temporary wielders for the other twelve zodiac miraculouses. 

Ideas for Alya and Nino in Season 3