Captain America's Shield
Captain America's vibranium shield is the iconic trademark weapon of the superhero also known as Steve Rogers. It has made regular appearances in the MCU ever since Cap first appeared in Captain America: The First Avenger - but a prototype version of it appeared twice before that, foreshadowing the coming of the super soldier to the franchise. In Iron Man, the prototype could be seen laying around in Tony Stark's workshop and, later, in Iron Man 2, it was seen again, when it was picked up by Phil Coulson and subsequently used by Stark to prop up his homemade particle accelerator.
Back when Edward Norton was still playing Bruce Banner, The Incredible Hulk introduced audiences to Samuel Sterns, played by Tim Blake Nelson. In the movie, Sterns attempted to help cure Banner of his Hulk problem, but ended up getting some of Banner's blood inside an open wound when Emil Blonsky attacked him. As a result, a visible mutation began to take place on his head - this was clearly a hint towards his future transformation into The Leader. The Leader is one of the Hulk's most formidable foes in the comic books and it would be great to see the psionic villain in a future movie. He has actually appeared in some of the MCU's spin-off comics, but is yet to emerge on film. Maybe that will happen in Phase Four of the franchise?
Muspelheim and Surtur
In Thor: The Dark World, the cosmic event known as the Convergence was taking place. It resulted in the alignment of the Nine Realms, causing the dimensional boundaries between them to become very thin. As the Nine Realms begin to converge in the film’s final act, a number of portals leading to them all are visible. One of the portals shows a flame-filled world with what appears to be a fiery body moving around inside it. The realm in question is Muspelheim - the home of Surtur and his army of Fire Demons - and the creature moving around inside it may well have been Surtur himself. Given that he is expected to be one of the main villains in the upcoming Thor: Ragnarok, this was surely foreshadowing his future appearance in the MCU.
Adam Warlock's Cocoon
Adam Warlock is a powerful superhero with strong links to the Guardians of the Galaxy in the comic books - but did you know that a recurring Easter egg has hinted towards him appearing in the MCU? In the mid-credits scene of Thor: The Dark World, audiences were given their first look at Benicio del Toro's character, The Collector. His museum was filled with rare artefacts and living beings - and one of the items that was visible in this scene was a large cocoon. It appeared again in Guardians of the Galaxy and, on closer inspection, it is clearly the cocoon that Adam Warlock uses for self-preservation and recuperation in the comic books. With that in mind, we can probably expect to see him in future MCU movies, which is a very intriguing and exciting thought.
In Ant-Man, Scott Lang's good friend Luis - played by Michael Peña - loved to tell a story. At the end of the movie, when he tells Scott about the fact that Sam Wilson is looking for him, he tells an elaborate version of events that includes references to heroes who jump, who swing and who can crawl on walls. All three of those could be interpreted as being references to Spider-Man, but the wall-crawling reference was definitely confirmation that he exists in the MCU. It was the first time he had been mentioned in the franchise, after Marvel Studios and Sony Pictures agreed a deal to bring him into it, which was a very exciting thing to hear.
The Winter Soldier's Trigger Words
In Captain America: Civil War, it was revealed that HYDRA's control over Bucky Barnes came in the form of trigger words that turned him into the brainwashed assassin known as the Winter Soldier. Some of the words included the likes of "longing", "freight car", "one", "nine", "seventeen" and "benign", but the one that was seemingly as Easter egg was "homecoming". This was undoubtedly a fun nod to the exciting news about title of the upcoming Spider-Man solo movie in the MCU - it's called "Spider-Man: Homecoming" and will be released in 2017, which is something we're definitely all looking forward to.