Attack on Titan is set in a world where humanity is believed to be on the verge of extinction because of the presence of giant, man-eating humanoid monsters called Titans. Eren Yeager is the main protagonist, and when he saw his mom get eaten by a Titan, he vowed to exterminate every single one of them.
Eren was a very good person, but after gaining his Titan Shifter powers and learning the truth about the world, he decided that the best course of action would be to trigger the Rumbling in order to destroy all of humanity. This change has shown just how much Eren has changed over the course of the series, but even he cannot accomplish such a feat by himself. It's true that he has an entire faction dedicated to his cause, but if he had the chance, he would work with several other anime villains who have the power to help him get the job done.
10 Aizen's Complete Hypnosis Would Come In Handy (Bleach)
If given the chance, many anime villains would choose to join forces with Bleach's Aizen because he is an immortal being with incredible speed and power. What really makes Aizen formidable is his Zanpakutō, Kyōka Suigetsu, which allows him to control all five of a person's senses.
If Eren was able to convince Aizen to help him, his plan would go a lot smoother because Kyōka Suigetsu can simply alter Eren's appearance. This would allow human Eren to move around without alerting the Scouts or Marley, and during fights, enemy Titan Shifters could be made to attack the wrong target while Eren attacked them from behind.
9 Vaccine Man Wishes To Wipe Out Humanity For His Own Reasons (One-Punch Man)
One-Punch Man has its fair share of respected heroes, but only Saitama was strong enough to stop Vaccine Man. This being was supposedly created thanks to all the pollution and waste caused by human beings, and he believes that it is his duty to save the world by wiping out all of humanity.
Eren wishes to destroy humanity as well, which means that their respective goals align. Eren may have several Titan powers and the Rumbling on his side, but Vaccine Man would guarantee his plan's success because he possesses unfathomable physical strength. He can also fire energy balls that can cause large-scale destruction.
8 Eren Could Offer Father Multiple Philosopher's Stones (FMA: Brotherhood)
Eren has managed to significantly ruin his likeability over the course of the series, but he would definitely sink even lower if he agreed to work with Father from Fullmetal Alchemist: Brotherhood. Father is the oldest and strongest Homunculus, and his main goal is to not be governed by the laws of the universe.
In order to accomplish this though, he requires one or more Philosopher's Stones, which can be created by sacrificing thousands of souls at once. Father would agree to work with Eren since he would be allowed to make as many Stones as he wished, and Eren would gain an ally whose alchemy powers essentially make him a god.
7 Lelouche Can Manipulate Others With His Geass (Code Geass)
Lelouche is the main protagonist of Code Geass, but like Eren, he turns himself into a villain in order to bring peace to the world. Lelouche is a Prince who wishes to take down his evil father, and when he meets a mysterious girl, he gains the power to do so.
This power is called Geass, and it allows Lelouche to enter the minds of others. This gives him the ability to plant commands into someone's mind, and it can work on a large group of people as well. If Eren decided to work with Lelouche, he can use the Geass to make everyone stand down while the Rumbling runs its course.
6 Satō Is An Immortal Sociopath With Great Tactical Skills (Ajin: Demi-Human)
In Attack on Titan, the Eldians want to live their lives without the fear of being attacked, subjugated, or persecuted, which is a plight that Satō can relate to. Satō is the main antagonist of Ajin: Demi-Human, and he acts like a terrorist in order to improve the status of every Ajin in Japan.
As an Ajin, Satō is an immortal being who can come back to life after being killed, and he can control an IBM, which is a black ghost that can cause significant damage. Satō has a great tactical mind that Eren can use, and he will make sure that the Rumbling happens because deep down he is a sociopath who enjoys killing and destruction.
5 Madara Could Wipe Out Everyone Who Stands In Eren's Way (Naruto: Shippuden)
Madara's ultimate goal was very different than Aizen's, but there are still plenty of villains who would join forces with the legendary Uchiha. In Naruto: Shippuden, Madara wanted a world where no child had to fight and die on a battlefield, and he wanted to accomplish this by trapping everyone on Earth in an infinite dream world.
If Eren worked with him, he could ask Madara to cast the Infinite Tsukuyomi to end the violence between Marley and the Eldians. If that didn't work, Madara could use his Uchiha might to bring peace to Eren's world by force. Madara has shown that he is capable of taking on an entire army by himself, and his perfect Susanoo would cut down an enemy Titan Shifter with ease.
4 Esdeath Would Be A Great Living Weapon (Akame Ga Kill!)
Thanks to her Imperial Arm, Akame Ga Kill!'s Esdeath is capable of manipulating ice to the point that she can plunge an entire continent into a small ice age. She is even able to freeze time for a short period. Esdeath doesn't really care about politics or geopolitical issues, as she simply believes that only the strong should survive.
The old Eren would never ally himself with someone like Esdeath, but the current Eren would, and she would probably become his greatest weapon. Thanks to her ice powers, Esdeath should be able to wipe out an enemy army single handedly, and Titan Shifters won't be able to get close to either of them.
3 Eren Can Put Gotou's Need To Kill Humans To Good Use (Parasyte)
Parasyte is a horror series that sees an unknown parasitic species appear on Earth, and they develop a need to devour other members of their host's species. Gotou is a very special case, as his body is actually composed of five different parasites, which makes him especially strong and dangerous.
Eren wouldn't have to worry about Gotou attacking him because he isn't really human thanks to his Titan powers, but he can make use of Gotou's need to kill. Gotou can ravage the ranks of the Scouts and Military Police while Eren continues with his plan, and Marley's forces won't do much better because Gotou kills without remorse.
2 Orochimaru Would Agree To Work With Eren To Fulfill His Own Selfish Needs (Naruto)
There are definitely villains who are worse than Orochimaru, but that doesn't mean he can't help Eren. Orochimaru is one of the most skilled ninja in the Naruto universe, and that is because he has spent years learning a myriad of powerful jutsu.
Thanks to all his experiments, Orochimaru has learned how to transfer his soul into someone else's body, which essentially makes him immortal. When Eren sees what Orochimaru is capable of, he will undoubtedly ask for the powerful ninja's help, and Orochimaru will agree because it will give him a chance to study the Titans and potentially steal Eren's body
1 Light Yagami Can Kill Key Targets With The Death Note (Death Note)
Light Yagami started off as an ordinary high school student, but that all changed when he found the Death Note. This notebook is a tool used by Shinigami, and when someone's name is written inside of it, they will either die a specific way or because of a heart attack.
Light is a natural genius who wanted a world where injustice didn't exist, but things eventually got out of hand. Eren could use Light's mind to come up with a way to fix the world without the Rumbling, and if no solution could be found, he could just ask Light to eliminate every high ranking threat with the Death Note.