Who should be the one to kill Thanos in Avengers 4?

10 years of the Marvel Cinematic Universe led to this very moment, in which Earth's Mightiest Heroes would unite against the biggest threat to the galaxy, the Mad Titan Thanos. In his quest to "save" the universe by erasing half of all life using the power of the Infinity Stones, Thanos has proven that he is the biggest, the baddest, and the most powerful opponent that both the Avengers and the Guardians have ever faced. In the words of Bruce Banner, "he invades planets, he takes what he wants, he wipes out half the population". In short, Thanos is a very bad dude, and Avengers: Infinity War showed us why he's a danger to the universe as he was successful in retrieving all of the Infinity Stones and wiping out half of all life with just a snap of his fingers. And, while we do know that the Avengers and the Guardians will eventually win and defeat the Mad Titan, the biggest question on everyone's mind is, who will be the one to end Thanos's reign of terror once and for all? 
Well, I've narrowed it down to 6 likely candidates who could potentially kill Thanos in Avengers 4: Iron Man, Thor, Star-Lord, Gamora, Drax, and Nebula. 
Let's start with Iron Man. Ever since the events of The Avengers, Tony Stark has been suffering from major PTSD since his near-death experience after the Battle of New York. He's always been fearful that another potential threat like the Chitauri will invade Earth again. This drove him to make numerous contingency plans in order to protect Earth, such as the Iron Legion in Iron Man 3. This almost ruined his relationship with Petter Potts, as he was so scared of both losing her and leaving the Earth defenseless against the Chitauri again. Skip ahead to Avengers: Age of Ultron, Tony decided to create the Ultron Program, which was meant to be a defense mechanism in case aliens invades Earth again, which would also mean disbanding the Avengers because they wouldn't be needed anymore. Unfortunately, thanks to the unknown alien properties of the Mind Stone, Ultron went rogue and almost destroyed the planet. While the Avengers saved the Earth again, it came at a cost. Tony's actions led to the creation of the Sokovia Accords, which would give the government total control of the Avengers. Tony and Steve were on opposing sides to this, which lead to a conflict within the Avengers, and eventually they disbanded. This left Earth defenseless against any potential threat. Now at Infinity War, Tony faces the biggest challenge yet in Thanos, a threat that he's prepared to face for 6 years. And while Tony gave it everything he got and even made the Mad Titan bleed, it wasn't enough to stop Thanos. And he had to witness half the universe die, including his friends, and even someone who he considers a son, Spider-Man. A threat that he waited 6 years for, and he couldn't even beat him. 
Next up is the God of Thunder himself, Thor. As the king of Asgard and one of the most powerful beings in the universe, Thor has fought countless battles and has always managed to come out on top, even at his lowest. Until he met Thanos. In one fell swoop, he witness this monster slaughter half of the remaining Asgardians aboard his ship and was powerless to stop him. Even the awesome power of the Hulk wasn't enough to stop Thanos. Thor was forced to watch Thanos kill Heimdall in cold blood, and eventually his own brother, Loki. After meeting the Guardians, Thor knew that he needed a new weapon in order to kill Thanos, and he, Rocket, and Groot went to go get it. But after a heartfelt chat with Rocket, we learned that while Thor was physically ready to fight Thanos, he wasn't mentally ready. Losing his mother to Malekith, his father to time, his sister to Surtur, his friends to Ragnarok, and his brother to Thanos, Thor truly was alone in the universe. Facing the heat of a dying star, Thor proved that he was willing to do whatever it takes in order to stop Thanos. And after getting Stormbreaker and heading to Wakanda, Thor managed to stab Thanos in the chest and almost succeeded in killing the Mad Titan once and for all, but as Thanos said, he "should've got for the head". Thor was forced to witness Thanos snap his fingers and complete his task of killing half the universe. Thor was forced to watch as he failed to both defeat Thanos and save the universe. 
Next up, Star-Lord. Though Infinity War was the first time they've met, Peter knew that Thanos was a threat to the galaxy, and he also knew that he was a threat to Gamora. Peter is willing to do whatever it takes in order to stop Thanos, even if it meant sacrificing the one he loved in order to save innocent lives. After capturing Gamora, Peter was more determined than ever to stop Thanos and save Gamora. And while they didn't see eye to eye, Peter and Tony knew that they had to work together so they can prevent Thanos from getting the rest of the stones. And even though Thanos was incapacitated, Peter was still determined to find Gamora. But after hearing the horrific truth from both Mantis and Nebula about Gamora's faith, it was too much for Star-Lord to handle, since he already his mother, his father, and Yondu. And having to dealwith losing her too was too much to handle, and he snapped on Thanos, allowing him to break free on Mantis's trance. This, unfortunately, led to him getting the Time Stone from Strange and heading to Wakanda to get Vision's Mind Stone. Peter was also forced to watch his fellow Guardians dust away, him included. This has lead to people blaming Star-Lord for Thanos's victory. And while he will be back in Avengers 4, the question is: can Peter redeem himself for letting Thanos win? 
Let's talk about Gamora, next. Gamora is Thanos's favorite daughter, and is truly the only one she loves. The feeling, while slightly mutual, is also mainly disgust and horror about the things that Thanos has done to both her and the galaxy. As a child, we saw Thanos take Gamora away from her parents when he slaughtered them, as well as half her planet. He trained her to become the Deadliest Woman In The Galaxy, a title she greatly rejects. And she was even forced to fight her own sister, Nebula, in order to prove who's more worthy of his love and affection. While Gamora always won the fight, she eventually lost her sister because of him turning her into a cyborg. Gamora still holds that guilt of not being able to help her sister, as well as the guilt of helping that maniac kill billions of innocent people. So, joining the Guardians of the Galaxy was her way of redeeming herself and she was able to become a hero alongside her new family. Knowing the location of the Soul Stone, Gamora knew that Thanos getting her would mean the end of the universe, which is why she forced Peter to kill her if necessary. After that fails, Gamora is forced to see Thanos torture Nebula in order to give up the Soul Stone. And while she knew that the Galaxy would be in danger, she couldn't let Nebula die. And Gamora was forced to make the ultimate sacrifice when Thanos killed her in order to get the Soul Stone. It's unknown if Gamora will be revived in Avengers 4, but if she is, she and her dad have a score to settle. 
Up next is Drax the Destroyer. Drax is basically a one man army, as has sole purpose in life is to avenge his family's death by killing Thanos. Ronan may have plugged the trigger, but Thanos showed him the way. Drax is more then determine to destroy the man responsible for killing his loved ones. And while he did have a chance in Infinity War to end it, he decided to use team-up instead of brute strength to fight Thanos. Drax's family's death still haunts him to this day as he still believes that they won't truly be avenged until Thanos is dead. But, after killing Ronan, he did feel a little bit more satisfactory knowing that one of the men responsible for killing his family was dead. And after meeting the Guardians, Drax has never felt more at home with his new family. He, the Avengers, and the Guardians almost won that day, but thanks to a distraught Star-Lord, Thanos broke free and caused total chaos. Unfortunately, Drax was another one of the snap victims, but we do know Drax will be hoping for a rematch. 
Lastly, Nebula. Her relationship with Thanos is.... complicated. She was taken from her homeworld and raised by Thanos too to become a fighter. Unfortunately, her sister bested her everytime, who eventually led to Thanos ripping off parts of her body to be replaced with mechanical enhancements. All Nebula wanted was a sister, and she felt that Gamora didn't care what happened to her. This caused a hatred between the two, as they have constantly fighting ever since. And while they do eventually make amends, the fact is that her life was destroyed because of Thanos torturing her and pinning her against her own sister. She was close to killing him, but was captured and tortured in order to convince Gamora to give him the Soul Stone's location. Nebula obviously didn't want to lose her sister, so she reunited with her fellow Guardians on Titan in order to stop that monster. Unfortunately, it was too late, and Thanos already killed Gamora in order to retrieve the Soul Stone. Saddened by the news, she told Star-Lord, not knowing that that's what led to Thanos escaping the Avengers and Guardians' trap and completely wiped the floor with them. Nebula managed to survive the snap, and was stuck on Titan alongside Tony Stark. 
And these are the main reasons why each character has a chance to kill Thanos in Avengers 4 next year. A long list, yes, but very informative. The decision is obviously up to you. Leave in the comments below who you think deserves the privilege of killing the Mad Titan once and for all.