2014 Thanos almost killed his timeline's Avengers in Avengers: Endgame to taunt the 2023 heroes. Thanos has been a looming presence in the Marvel Cinematic Universe for years, but it was really in the mid-credits scene for The Avengers in 2012 that he was touted to be the biggest threat the heroes could face. It took six years, but Thanos and the Avengers finally came face-to-face in Avengers: Infinity War with devastating results for the heroes.
Thanos was successful in gathering all six of the Infinity Stones, snapping his fingers, and wiping out half of all life across the universe. He took pleasure in this victory and reduced the Infinity Stones to atoms so that the Avengers couldn't undo his actions. After Thor killed him and a five year time jump, the team assembled again for a last-ditch attempt involving time travel. However, their plan resulted in the 2014 version of Thanos learning of their scheme thanks to Nebula's memory network. He was able to kidnap 2023 Nebula, replace her with 2014 Nebula, and arrive on Earth in 2023 with a massive army.
Before Thanos' ship shot out of the enlarged Quantum Tunnel, though, the Mad Titan almost took a detour to his Earth in 2014. Longtime MCU writers Christopher Markus and Stephen McFeely, who wrote Avengers: Infinity War and Avengers: Endgame, held their own panel at San Diego Comic-Con 2019. During the panel (via CBR), Markus revealed details of an alternate version of the script where Thanos killed the Avengers again.
"In one, he - being Thanos - knowing it would take Nebula some time to rejigger the time machine... so he went to his present-day Earth and wiped it out and killed the Avengers, and then Nebula would turn on the time machine, he would walk through the time machine with 'the body of an Avenger he had killed' and say, 'I killed you again. What are you going to do?'"
One of the complaints that did come out of Avengers: Endgame is that the 2014 version of Thanos was not as intimidating or complex as the one audiences saw in Avengers: Infinity War. The possibility of him killing all of the Avengers in 2014 before facing down what was left of them in Avengers: Endgame would certainly make his encounter more terrifying for fans. However, the feeling of dread would probably be felt more by the actual heroes, as he would have the body of one of the Avengers with him and taunt them with the fact that Thanos has once again won.
While Thanos killing the 2014 Avengers would've been a wild additional sequence for Avengers: Endgame to include, it still would not have changed the finale. There could've been some great parallels to the vision Tony Stark had in Avengers: Age of Ultron, though; and this sequence may have also made the portals scene all the more powerful. Captain America would be the last one standing in front of a Thanos who was on the verge of killing the Avengers again, so the sudden reinforcements would be a huge moment of relief for him and audiences. Either way, Thanos wasn't going to win Avengers: Endgame, but seeing him kill the Avengers earlier would easily be one of the more memorable moments of the whole movie.