Thanos has always been one of Marvel's deadliest threats, a Mad Titan capable of bringing down almost any threat thrown his way. He's come close to wiping out all of reality multiple times, only ever held at by the likes of Captain America and the Avengers. But in at least one reality in the multiverse, Thanos found a way to turn those enemies into his greatest allies.
In What If: Infinity - Thanos by Joshua Williamson and Mike Henderson, Thanos actually ended up joining the Avengers -- and more or less replaced Captain America as their leader.
In the core-Marvel Universe of Earth-616, Infinity saw the Avengers take to the cosmos to help various galactic empires in their battles against the Builders. A powerful race of ancient aliens who'd turned their attention to the beings who'd come to grow across the galaxy, the Avengers became chief members of the resistance against them. However, their campaign into space left Earth relatively unprotected, allowing Thanos and his Black Order to invade the planet and bring massive destruction to the world. He did this all in an effort to find and kill the last of his offspring, Thane -- who'd been living on Earth in secret for almost his entire life.
However, in the reality of Earth-15121, Thane wasn't hidden on Earth. Instead, he had been taken by the Builders, as this variant of Thane was himself connected to the ancient race. This prompted Thanos to forge an "alliance" with the unified empires of the galaxy, convincing them that he sought to end the Builders before they could bring about a radical form evolution of life across the galaxy. As such an act would wipe out countless innocents while also preventing Thanos' beloved Death from taking the universe, Thanos argued their missions aligned enough for a partnership. Desperate, Captain America reluctantly agreed to the plan. With the aid of Thanos and the Black Order, their war against the Builders proved to be far more successful than in most realities.
Thanos quickly gained the trust and confidence of many Avengers, notably Thor -- with the pair bonding over their friendly contests on the battlefield to see who could kill more Builders. But despite this, Captain America refused to address Thanos as an Avenger. Pushing the offensive, Thanos led a group consisting of Captain America, Captain Marvel, Thor, Manifold, Gladiator into battle against the Builders' command ship. Following Thanos into the depths of the ship on his own, Captain America discovered the Mad Titan and found Thane -- and preparing to kill his son. The Avengers leader refused to stand down -- resulting in a battle that Steve Rogers had no hope of winning. When Thanos returned to the battlefield, he saved Thor using the burnt and bloodied shield of Captain America, claiming Rogers had died a "hero's death" killing the last of the Builder Commanders. Although Thor doubted his words, he and Captain Marvel relented when Thanos laid down his arms and argued that they could kill him if they wanted -- but that there were plenty of other worlds left to save. Thanos returned to Earth triumphant, with the Avengers keeping his role in the conflict a secret and seemingly having come to trust the Mad Titan.
But even beyond them, the Illuminati of this world -- dealing with the Incursions that resulted in the events of Secret Wars -- were confronted by Thanos, who demanded a seat at their table to help lead them in their mission to "kill worlds." It's a dark reality, one with the potential of heroism still alive but in the very bloodied hands of Thanos. And all it ultimately took to cement his place as the new figurehead of the Avengers was to kill the old one.