In 2008’s “The Incredible Hulk” Tim Blake Nelson's Samuel Sterns was left on the ground with a bubbling head after a few drops of Hulk's blood falls on his wounds. It seemed to set in motion the beginning of the villain The Leader, but unfortunately, plans for a sequel were canned after the film’s poor box office showing.
Jane’s humorous intern Darcy provided great comic relief amid all the action and romance, and in "Thor: The Dark World” Darcy finally got some romance herself with her assistant Ian Boothby. However, when Darcy returned in "WandaVision" there wasn’t any mention of Ian after their climactic kiss in “The Dark World.”
In "Captain America: Civil War," Tony Stark tells Steve Rogers that he and Pepper Pots are taking a break and it was a significant development that played into the darker headspace of Tony in “Civil War.” However, this development was abruptly upended in "Spider-Man: Homecoming," when Pepper and Tony were back together, as if nothing had ever happened.
In "Thor: The Dark World" scientist Erik Selvig struggled to maintain a proper psychological state after succumbing to Loki’s mind control in “The Avengers.” But when Thor recruits him to stop Ultron in “Age of Ultron” not only had he returned to his regular day job, there wasn't any indication of him having any psychological trauma.
At the end of "Iron Man 2," Justin Hammer while being carted away by law enforcement he calls out to Pepper Potts that he'll be getting his revenge sooner, rather than later. MCU films have a knack for keeping their villains alive, but for whatever reason, Hammer and his revenge plans have yet to be seen.