Rhodey is shown being friendly with Steve, Sam, and Nat, as well as fighting with Thunderbolt Ross and expressing regret over the Sokovia Accords. Did he still generally believe in the Accords yet simply decided to suspend this belief for the sake of having everyone on board to help save the world while dealing with the frustration of Ross' insistence on following the rules, or has the time he's spent working under the Accords made him realize how ineffective they are and that Steve was right in their problematic use?
Rhodey is also friendly towards the Vision despite the latter firing the shot that crippled him in Civil War. A case of Rhodey turning the other cheek, or considering it his own fault given that he himself ordered the Vision to take the shot? Or did he just accept that accidents such as this happen a lot on the battlefield and there's really nobody to blame? Given that it's implied Rhodes and Vision spent most of the intervening 2 years with Tony in the Compound, it's likely they've had time to work out any guilt or resentment.
When they realize Vision is missing, Tony admits that Steve Rogers would probably be able to find him. Was this acknowledgement of Steve's skills by believing he could track him, or was he aware of Vision and Wanda's affair, while pretending to not know about it for their sake, and so thought Steve would be able to find Wanda, and thus Vision? Or even, did he believe that Vision disabling his transponder was him joining Steve's side completely? (Word of God on the DVD commentary says it's the second one; Tony knew that Vision was in contact with Wanda behind his back but turned a blind eye to it, and that Steve would have some idea where they were.)
Was Thanos's love for Gamora completely unconditional and his need to sacrifice her to obtain the Soul Stone something that caught him off-guard? Or was he at least somewhat (if not completely) aware that the Soul Stone required a sacrifice of a loved one the entire time, and thus orchestrated having a 'loved one' from the start? It is possible he took Gamora and Nebula, pitted them against one another in order to feel pride for the superior fighter, which would eventually turn into tenderness and love over time all so he would have the key required to obtain the Soul Stone once located.
Seeing as he's a One-Scene Wonder, it's left ambiguous whether Red Skull has reformed (or at least given up on his ambitions) during his long stint on Vormir, or if he's still just as evil as he was in the 1940s yet is unable to do as he wishes.
Does Thanos completely believe that the solution for his planet applies to the entire universe? Or is this psychological projection on his part? In other words he's trying to validate his theory with his own planet by testing it out on other species; that way he won't have to live with the guilt of his planet's downfall, and he can feel validated that if they had simply gone with his plan then they would have lived. Furthermore, given the powers granted by the stones, Thanos could easily find better and humane alternatives such as creating infinite resources to sustain the population, yet still remains dead set in his goal to wipe out half the universe. Was it a case of Insane Troll Logic, or does he feel that he's already sacrificed so much to turn back now, and finding another solution would mean that he has to come to terms with the fact that he had slaughtered billions if not trillions of lives for nothing? Thanos has one confirmed case of his plan leading to his intended result — Gamora's home planet. As such, he thinks that if his plan worked there, surely it will work on other planets, right? Therefore, it is either he genuinely believes his culling is the only possible solution to the universal problem of finite resources or he refuses to consider any other solution even with the power of the Infinity Gauntlet.
DVD commentary by the Russo brothers lean towards the former interpretation. Thanos's methods is less a desire to prevent an Overpopulation Crisis, but to prove that his proposal that was rejected by his people was right all along.
The idea that Gamora's home planet is a confirmed case of Thanos's plan working is itself in doubt. Gamora's rap sheet during Guardians of the Galaxy indicated that she was the last of her people. Did halving their population cause the entire race to eventually die out, leading to them no longer using resources at all and Thanos therefore considering them a "success"?
Among those who are killed at the end with the fingersnap, two were separated from their lovers in the course of the film — Quill and Wanda. To both of those, Thanos showed a sort of Pet the Dog moment — telling Gamora that he "likes" Quill and patting Wanda on the head after she had to kill Vision. Did he disintegrate them as a Mercy Kill so they could be with their lovers? Wanda at least has an expression of relief on her face when she goes.
This interpretation itself raises an important question: did Thanos exercise any control over who was vaporised by the snap? In addition to the aforementioned (possible) Mercy Kill, it's been pointed out by fans that Thor and Tony's survivals can be seen as Thanos keeping his word to Loki and Strange to spare their lives in exchange for the Space and Time stones. On the other hand, Thanos's stated philosophy is for the process to be "random, dispassionate, fair for rich and poor alike"; picking and choosing individuals to live or die invalidates that philosophy and makes Thanos a hypocrite. Besides which, the snap did not spare races that Thanos had already decimated, as Drax, who mentions in the film that his homeworld fell to Thanos, is one of the victims, and Word of God specified at least two Asgardians as victims as well; if Thanos was being at all specific about who he wanted dead, he surely would have spared species whose population he'd already halved.
Is Doctor Strange telling Tony "there was no other way" an attempt to give Tony hope that Thanos can still be stopped, and the omnicide was simply all part of the plan and reversible, or to apologize since he knew in advance that Tony would have to watch the rest of them die while being left behind to deal with the consequences? Endgame only adds another option: is this Stranges way to apologize in advance for the Heroic Sacrifice he knows Tony will have to perform to save the Universe? He is a doctor, after all, and he knows hes condemning a man to death for the sake of everyone else.
Both Wanda and Steve refuse to even consider sacrificing Vision in order to destroy the Mind Stone. Was their choice heroic and their resolve of "we don't trade lives" something to aspire to, a clear contrast to Thanos's principles that included sacrifice of any kind to achieve his goals? Or was it selfish and hypocritical of them, considering that Vision himself wanted to be sacrificed to destroy the Stone, and that countless Wakandans died while attempting to save one man's life, even before the universe was halved?
When Thor mentions that his best friend was stabbed through the heart, was he speaking of Heimdall, or Phil Coulson?
The movie has a continuing accidental theme of Love Is a Weakness. The heroes continuously fail to stop Thanos because of their love/compassion for other characters, from Loki, to Gamora, to Peter Quill, and ultimately to Scarlet Witch at the end. At one point Thanos himself is faced with this dilemma when he realizes he has to kill Gamora, his favorite adopted child, to get the Soul Stone. Ultimately, Thanos goes through with it because his goal is far more important, and indeed it is him who ultimately comes out on top. Had the heroes actually been willing to make necessary sacrifices instead of trying get the perfect solution all the time, and not let their emotional attachments control them, Thanos' plans could have possibly been stopped.