Thanos has quite a few children, Gamora and Nebula being the best known thanks to their Marvel Cinematic Universe appearances. But there was another who was more powerful than both of them. And he's arguably more dangerous than even Thanos.
During an encounter with a lost Inhuman tribe, Thanos conceived a child with one of the Inhumans. He didn't know about the boy who would be called Thane until he was nearing adulthood. But once he did, Thanos realized the boy could pose a major threat to him. Thane proved to be all of that and more, his powers almost godlike. His story ended soon after his ascension in the comics, but Thane could still make an appearance in the MCU. As to whether that will be as a hero or villain is anyone's guess.
Thane first debuted in New Avengers #10 by Jonathan Hickman, Mike Deodato Jr., and Jerome Opeña. He became aware of his lineage when Thanos began a war of conquest to find his missing child. It was Black Bolt who learned of Thane's existence and status as an Inhuman, thus he sent the Illuminati to find Thane before Thanos did, fearing what could happen if the two allied or fought. Doctor Strange found Thane first but was controlled by Ebony Maw to forget he had located Thanos' lost child.
Black Bolt was forced to battle Thanos and in the process detonated a terrigen bomb that spread the gas all over the planet. Anyone with inhuman DNA was subjected to terrigenesis, which included Thane. Transformed, Thane's powers killed an entire town before Ebony Maw gave him a suit to control his powers and then proceeded to contain him. Drawing Thanos to them, the Mad Titan revealed his intent to kill Thane, prompting the young man to accept his power and imprison Thanos in a state of living death within amber.
Thane tried to remain in isolation after that, becoming the protector of a group of outcasts. When they were murdered, however, Thane found himself back under Ebony Maw's tutelage, becoming convinced of his destiny to be the ruler of the universe. This would inevitably lead to a final confrontation with his father. Their battle decimated the planet they were on and Thanos ended up depowering his son and leaving him trapped inside the god quarry.
Even though his time was short, Thane's power proved vast. True to his lineage, Thane possessed a death touch in his left hand and the ability to trap people within an ambered state of living death with his right. His one weakness was that he had no true control over any of these abilities. Without his suit, those powers would spiral out of control and hurt or kill everyone near him.
Thane himself is very much a victim of circumstance. He did not ask to be created nor changed. But the very fact of his existence made him a pawn or threat to major powers of the universe. And Thane has proven himself very susceptible to manipulation. Should Thane make an appearance in the MCU, it's possible that he may become a threat to everything by accident rather than choice. After all, there is a lot of time Thanos spent offscreen, who knows what he got up to during that time? Or with who.