In fanfiction, when Eren meets Levi in the underground, they might initially have a tense encounter. Levi could be suspicious of Eren's motives. For example, he might question Eren's presence there as the underground is a dangerous and lawless place. Eren, on the other hand, could be in awe of Levi's skills and reputation.
Well, they might have a deep conversation about their different views on the world. Eren could try to convince Zeke of his more radical approach to saving the Eldians, while Zeke might push back with his own ideas of euthanasia for the Eldians. It could be a very tense encounter.
Gabi remains alive in the manga. The plot doesn't lead to her death. It's one of those character arcs where she continues on despite facing various challenges.
They could start to plan a strategy to deal with the Marleyans together. Since both have their own experiences with Marley, they might come up with a complex plan involving Titan powers and political manipulation. For example, using Zeke's position as a Marleyan weapon and Eren's influence within Paradis.
For a more villainous touch, including Voldemort or his followers could create high - stakes situations. Bellatrix Lestrange could be a terrifying adversary for Harry and Gabi. On the other hand, adding new original characters, like a powerful but reclusive wizard who lives in the Forbidden Forest, could also make the story more exciting.