Thanos' snap plan in Avengers: Endgame - with his perverted sense of altruism leading Thanos to genuinely believe he was helping the universe via his snap.
While Guardians of the Galaxy, it was not until Infinity War that the Mad Titan's plans finally became clear. Rather than simply wanting to become the strongest in the universe or take over the world like many villains in the superhero genre, Thanos instead wanted to wipe out half of the universe under the guise of helping the universe achieve balance. Infinity War's ending sequence, in which Thanos succeeds in his plan, is one of the most remarkable moments in MCU history - but the Avengers had encountered a similar challenge before they even knew about Thanos.
In the second Avengers movie, Very much like Thanos, Ultron wanted to cause a great amount of death so that a new, better life could emerge on Earth. Obviously, Ultron's plan was on a much smaller scale – only the people of Earth would be affected by Sokovia hitting the ground – but it is still very similar to Thanos' snap, at least in concept. Both believed their terrible actions were for the greater good, and they did not care about the number of casualties inflicted as long as they could prove their point. Ultron's plan was put together much quicker than Thanos' and would never have the same effect as the snap did, but that does not change the fact that the Avengers had to stop two antagonists from trying to reset life twice in a row.
The idea of a villain trying to cause destruction so the survivors can grow stronger is nothing new in the wider superhero genre outside of the MCU. Even grounded villains such Arrow season 1 had similar plans for Gotham and Starling City – once again on a much smaller scale. The fact that this type of story is almost a supervillain trope makes MCU's Thanos even more remarkable, as his storyline was able to stand out from all the others in the genre despite its lack of originality. This, in turn, helps explain why the Ultron comparison is not brought up frequently against Thanos despite the characters' similarities - and Thanos' plan undoubtedly had a much bigger impact both on audiences and within the MCU.
Thanos managed to cause countless deaths in service to his idea of balance, and Ultron would have achieved something very similar in Age of Ultron had it not been for the Avengers' intervention. The last two Avengers villains fell into a relatively old villain's plan trope, but that does not detract from the quality of their stories - especially in Thanos's case. For years to come, Avengers: Infinity War's snap storyline will be a reference for stories with villains who believe they should be the leaders of a new life, despite the fact Marvel Studios already used Thanos' plan three years before his grand MCU entrance.
