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Throughout the Infinity Saga, Thanos proved his status as a being capable of wreaking havoc across the universe. However, there are some characters that cause even the Mad Titan to tremble in fear. While the MCU has moved beyond the Infinity Saga and introduced powerful new rivals like Kang the Conqueror, and more recently establishing the threat that is Doctor Doom, Thanos remains a stunningly powerful rival.
Even when the entire Avengers lineup tried to beat Thanos back, he was able to easily resist their efforts in order to gain the Mind Stone. Before that, when he had only collected the Power Stone, he was able to make short work of Thor, Loki, and the Hulk. But despite his spectacular powers, Thanos definitely has some characters who could pose a serious threat to his master plans.
While the comics present literally dozens of characters that could easily whoop Thanos, the talent pool of characters in the MCU is a little more limited. And yet, there are some characters who would almost certainly be too much for Thanos to handle.
Galactus
In the comics, Thanos has been aware of the immense and overwhelming presence of the World-Eater, Galactus. While Thanos is a Deviant-Eternal Hybrid, born from the power of the Celestials, Galactus is a vast being with powers akin to a Celestial, and by extension, his powers far exceed Thanos. As a result, Thanos tries to avoid crossing paths with this powerful devourer of the universe, and instead prefers to deal with rivals that he can more easily dominate.
In the MCU, Galactus has only appeared in The Fantastic Four: First Steps, a film which came out several years after Thanos was defeated in Avengers: Endgame. However, it does seem as though Galactus could potentially cross the multiverse and eventually make an impression in another reality. If Thanos were to ever stumble across Galactus, it is almost certain that he would struggle to match their power.
For this reason, Thanos is unlikely to willingly challenge Galactus, choosing instead to deal with mere mortal beings.
Odin
Continuing along this line of thinking, Thanos would also be fearful of standing up to a fully powered God. While Thor is the god of Thunder, and his adopted brother Loki possessed powers that could rival those of any Asgardian, both failed to stand up to Thanos. But, their father, Odin, is one of the most powerful gods in the entire MCU, which would mean he could have defeated a monstrous tyrant like Thanos with relative ease.
More than that, while Odin was at the height of his power, wielding the Odinforce and conquering multiple groups across the Nine Realms, he was clearly equipped to be able to overthrow massive armies of powerful creatures, which would have easily prepared him to take on any battle with Thanos and his Black Order.
Unfortunately, in the MCU, Odin had already reached the end of his impossibly long life by the time Avengers: Infinity War rolled around, which meant he wasn't there to step in and save his son Loki, or to guide his heir, Thor, in how he could effectively destroy the Mad Titan.
Franklin Richards
Another character who was introduced to the MCU in The Fantastic Four: First Steps is the young child of Reed Richards and Sue Storm. Being born of two powerful heroes, Franklin Richards was bound to have impressive abilities, but no one could have predicted the extent of his gifts until his parents met Galactus, and the World-Eater sensed the Power Cosmic within the unborn child.
By the end of the film, Franklin was born, and despite the appearance of being a normal baby boy, when tragedy struck and his mother passed away in an effort to save his life, Franklin was able to use his incredible powers to bring her back from the dead. But according to the comics and the confirmation from Galactus, this is just scratching the surface of what Franklin is capable of.
In the comics, Franklin has powers that can literally reshape reality to his will. Similar to how Thanos was able to wield the Reality Stone to create illusions, but Franklin's powers are innate, and literally able to bend and warp the laws of reality to his will. Inevitably, this would have been something that could have stopped Thanos in his tracks, but unfortunately, Frankly was still yet to be born, and arrived a few realities away on Earth-828.
Adam Warlock
Adam Warlock first appeared in the MCU in a post-credits scene of Guardians of the Galaxy Vol. 2, and while his return did not play out until Guardians of the Galaxy Vol. 3, which was released in 2023, several years after Thanos was defeated, he remains an intriguing character who had a lot of potential to be a rival to Thanos.
In The Infinity Gauntlet comic, the story that was adapted for Avengers: Infinity War and Avengers: Endgame, Adam Warlock is a central figure in the entire sequence. Interestingly, similar to how Vision is shown to be powered by the Mind Stone in the MCU, Adam Warlock is a being who derives power from the Soul Stone, after he was originally infused with the Stone by The High Evolutionary.
Now, in the MCU, Adam Warlock was created by another race of aliens, and he had no direct association with the Soul Stone, but seeing where the character's story originates, it's easy to see how this could have been connected, and if the MCU chose to adapt it, it may have been a very different story that played out in Avengers: Infinity War.
Scarlet Witch
Finally, one character who stood against Thanos and lost during the events of Avengers: Infinity War has increased their power exponentially since their confrontation, and would now likely inspire overwhelming fear for Thanos. Wanda Maximoff was one of the latest recruits to the Avengers team when the events of Avengers: Infinity War played out, and unfortunately, her lack of control over her powers led to incidents that put her on the backfoot.
Despite this, when Wanda came face to face with Thanos at the end of Avengers: Infinity War, fueled by intense grief at losing her beloved Vision twice, she was able to hold back Thanos, despite him having five of the six stones already. Her untrained raw power was clearly formidable, and even without refinement, Thanos appeared to hesitate when he saw her immense strength.
After the devastating loss of Vision, Wanda embraced the darker side of her power, and she became known as the Scarlet Witch. Her powers extended to allow her to warp reality and send her mind into other dimensions, with her almost getting to a point where she fractured walls between the multiverse and caused irreparable damage.
Presumably, at this point in her journey, the Scarlet Witch would have been an incredibly terrifying opponent for Thanos to try and fight against. Fortunately, this played out in projects like WandaVision and Doctor Stranger in the Multiverse of Madness, some time after Thanos was defeated. And ultimately, Wanda sacrificed herself when she saw what she became.
Across the board, there are many people who could present a real threat to Thanos in the MCU and beyond, and considering how ambitious and determined he was to achieve his goal of eliminating half of all life in the universe, he would have avoided clear dangers that could have put a stop to his plans.
