Ok, so I had this idea in my head about years ago before Avengers even happened. It was like the last hour of the 3rd one would be Red Skull & Loki & Thanos & Ultron (I worked Ultron into this theory when I found out) leading everyone of their armies into Earth and Hydra invading with them. Hydra Soldiers riding with the Chitauri. Asguardian warriors also riding with. And we have this huge climactic battle that last an hour of awesomeness. Ultron will be shutting down most of the electricity and shit and like fucking shit up that way. The Earth will be in a disaster because The Avengers can't be everywhere at the same time. (This fight will last 30 minutes) Cap will be fighting Red Skull. Iron Man will be trying to trigger all of his suits to his aid but Ultron in his way. Thor is trying to fight Loki but Hulk is in the way because Hulk is under mind control. The rest are doing something. Ant-Man is with Iron Man btw. SHIELD is fighting all in different places. (The scene will continuously switch from all of them) This might as well be the whole movie.
All of a sudden! BOOM! Loki tries stabbing his staff (Which has a blade at the end... I think) through Thanos. But Thanos will kill him and Thor will downright attack him. Hulk will slowly get up because Thor finally got him to stop his stupid shit. And Thanos will have Thor bow down. But Hulk attacks Thanos! Iron Man & Ant-Man stop Ultron from controlling everything and they come face to face with him leading some warriors. Iron Man sends all of his suits to destroy only CHITAURI, HYDRA, and ASGUARDIAN WARRIORS. But then, we see a portal opening up and Thanos looks at the screen. It's the Nova Corp. with the Guardians of the Galaxy and Heimdall, Sif, and the Warriors Three. Cap gets the upper hand on Red Skull when the Winter Soldier finally beats The Reaper. SHIELD starts whooping ass. We do this awesome scene where every hero starts showing up. Ghost Rider is riding up a building and like whipping people with his chain. Namor does this epic Superman Punch-esque throw of the Trident when a Chitauri "Whale" goes over the ocean and it badassly destroys it. We see Panther ripping shit up in Wakanda. We see pretty much everyone. Have Marvel pay a million dollars to Sony or even more to have just an epic cameo. Same with X-Men & Fantastic Four. PLUS THE FUCKING DEFENDERS BREAK OUT ON THE SCENE. Just think about all of this.