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.
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.
In this fanfiction scenario, Eren and Levin meet in the underground. It could be that the underground is a place where outcasts or those with special knowledge gather. Eren, with his strong will and determination, and Levin, with his own unique skills or knowledge, cross paths. They might start off on the wrong foot, with misunderstandings. But as they spend more time together, they discover that they can learn from each other. Eren could learn new ways of thinking from Levin, and Levin could be inspired by Eren's tenacity. And together they could embark on a journey to uncover secrets that could change their understanding of the entire world.
Their relationship might develop slowly. At first, they'd be suspicious of each other. But as they face difficulties in the underground, like escaping from traps or fighting off strange creatures, they'd start to rely on each other. Eventually, they could become friends who trust and support one another.