Armin could be another good character for this. He's intelligent and good at strategizing. In a 'Attack on Titan meets FNAF' fanfiction, he could use his wits to try and understand the nature of the FNAF elements and come up with plans to either avoid or combat the threats that come from the combination of the two universes.
Frankie Stein might also interact well. Frankie is always curious about new things and how they work. The FNAF animatronics with their mechanical and somewhat mysterious workings would be right up Frankie's alley. Frankie would probably try to study them and figure out how to make them less menacing, or at least understand their purpose.
Probably Leonardo from the Turtles and Freddy Fazbear. Leonardo is the leader, so he would likely be the first to step forward and assess the situation when they encounter the FNAF animatronics. Freddy, being the main animatronic, would also be in the forefront. Their personalities are quite different, with Leonardo being more disciplined and Freddy having a more mysterious and somewhat menacing presence. Their first interaction could be a tense stand - off as they try to figure out each other's intentions.
The characters from Pokemon, like Ash, would be very curious about the Titans. He might try to understand them first instead of immediately attacking. Meanwhile, the characters from Attack on Titan, such as Mikasa, would be initially cautious of the Pokemon. But as they see the Pokemon's powers, they would start to trust and work with them.
Eren and Raleigh would be likely to team up. Eren has his powerful titan - shifting abilities which could be useful in close - combat situations against both titans and Kaiju. Raleigh, with his experience in piloting a Jaeger, could guide Eren on how to better use his powers in coordination with the large - scale technology of the Jaegers. They could form a strong bond as they both have a strong sense of justice and a desire to protect their respective worlds.
Luffy would probably be really excited to meet the Survey Corps. He'd see them as cool warriors and want to team up right away. He might even try to convince them to join his crew. Eren, on the other hand, would be a bit more suspicious. He's used to a world full of danger and betrayal, so he might not trust the Straw Hat crew immediately. But as he sees their loyalty to each other, he could start to warm up to them.
Well, Naruto is a very empathetic character. So when he meets Annie, he would probably try to understand her motives for her actions in Attack on Titan. He might think that there is some good in her, even though she has done bad things. And when it comes to the interaction between Naruto's friends and the Attack on Titan characters, there could be a lot of cultural shock. The ninja way in Naruto's world is very different from the survival - based mindset in Attack on Titan. But they could also learn from each other. For example, the Attack on Titan characters could learn about teamwork from Naruto and his friends, while Naruto's group could learn about the importance of adaptability from the people in the Attack on Titan world.
The 'Attack on Titan' characters would be quite shocked by the technology and the general chaos in the 'Gintama' world. Characters like Armin would be very interested in all the strange inventions and the way society functions in 'Gintama'. He might try to understand the economic and political systems there. And the 'Gintama' characters would be in awe of the 'Attack on Titan' characters' ability to fight Titans. They could team up to face common enemies or just have a series of comical misunderstandings that lead to all sorts of adventures.
Eren could be really interesting. Given his ability to transform into a Titan, he might be curious about Godzilla and try to understand his power. He could also see Godzilla as a rival in terms of power and might try to prove himself by interacting with Godzilla in some way.
Death from DarkSiders and Black Widow could have an interesting interaction. Death has seen countless deaths and has a very different perspective on life and death compared to the human Black Widow. Their conversations could be deep and thought - provoking as they work together on a mission.
I think Mirabel from Encanto and Aang from Avatar would be likely to interact. Mirabel is all about finding her place in the family and Aang is on a journey to save the world. They could share their experiences of feeling different and having big responsibilities.