The Kingpin might be a good choice. He could see the Titans as a new weapon to use in his criminal empire. He could form an alliance with some of the more corrupt elements in the Attack on Titan world, like some of the military commanders who are only interested in power. He would use his wealth and influence to control the Titans and try to take over both universes. Spider - Man and the heroes from Attack on Titan would have to fight against his well - organized and well - funded forces.
Perhaps a new character could be created as the main antagonist. A mad scientist who has studied both the Titan world and Spider - Man's world for years. He discovers a way to merge the powers of the Titans with some of Spider - Man's enemies' powers. For example, he creates a Scorpion - Titan hybrid. This new character would be obsessed with chaos and destruction, believing that by destroying both universes and starting anew, he can create a 'perfect' world. His actions would force Spider - Man and the characters from Attack on Titan to join forces to stop him.
One possible plot could be that the Titans somehow cross into Spider - Man's universe. Spider - Man initially mistakes them for some new kind of giant robots or mutants. He tries to fight them using his web - slinging and acrobatic skills. But he quickly realizes they are much more difficult to defeat as they keep regenerating. Meanwhile, some of the Survey Corps members from Attack on Titan also cross over. They team up with Spider - Man, teaching him about the Titans' weak points at the back of their necks. Together, they fight to keep both universes safe from the Titans.
Dr. Octopus could be a great main antagonist. He could team up with a female super - villain from the Totally Spies universe. Together, they could create a plan to steal some top - secret technology. Dr. Octopus uses his mechanical arms to cause chaos in the city, while the female villain uses her own unique powers to keep the Totally Spies and Spider - Man at bay.
Armin could also be a main character in such a fanfic. He's known for his intelligence. As a permanent titan, he could use his smarts in unique ways. He might try to figure out the secrets of the titan form from the inside and communicate with the other characters in unexpected ways. For example, he could leave clues or messages for his friends using his titan abilities.
Eren would definitely be a main character. His ability to transform into a Titan makes him a unique element in the crossover. He could interact with the Transformers in interesting ways, like learning from their technology or using his power in combination with their weaponry. Optimus Prime would also be a main character. His leadership and strong moral compass would be crucial in bringing the two groups together. He could inspire the characters from Attack on Titan to see a bigger picture and fight for more than just survival. Mikasa would be another key character. Her loyalty and combat skills would be valuable in the fight against the combined threats that might come up in the crossover.
It could be a combined form of several of Spider - Man's classic villains. For instance, Doc Ock, Electro, and Sandman fused together by some dark magic. This new super - villain would have the intelligence of Doc Ock, the electrical powers of Electro, and the shape - shifting abilities of Sandman. Spider - Man would have to use all of his skills and perhaps some new ones he gains during the crossover to defeat this powerful adversary.
Doctor Octopus is another option. His mechanical arms and high intelligence could make for a formidable foe. He might see Deadpool and Spider - Man's partnership as an obstacle to his grand plans. He could try to outwit them and trap them in his elaborate traps using his scientific knowledge.
The Green Goblin could be a great main antagonist. He has a long - standing feud with Spider - Man. Maybe in this fanfiction, he could be out for a more brutal revenge after a particularly humiliating defeat. He could target Spider - Man's loved ones or try to completely destroy his reputation in the city.
The Green Goblin could be a great main antagonist even in a Spider Man reborn story. Maybe in this version, he has developed a new form of insanity - inducing gas that affects Spider Man's new powers. Another possibility is Doctor Octopus. He could have found a way to merge his mechanical arms with some sort of alien technology, making him far more powerful and a real threat to Spider Man reborn.
It could be a corrupted version of an old ally. For example, a former S.H.I.E.L.D. agent who was exposed to some strange space radiation. This has turned him evil and he now wants to use his knowledge of Earth and his new - found space powers to create chaos. He blames Spider Man for not being able to save him from the radiation and is determined to make Spider Man pay, all while causing destruction across the galaxy.
Perhaps Doctor Octopus. But instead of just having mechanical arms, his mutation gives him the ability to control any metal object in the city. He wants to take over the city using his newfound powers and sees Spider - Man as the only obstacle.