1. Don't Leave Tony And Steve As Friends At The End Of Avengers: Age Of Ultron
This one still beggars belief. With the next film featuring both characters set to be Captain America: Civil War, and an open discord already existing between the two during the events of Age of Ultron, it would have made perfect sense to leave their relationship fractured at the end of AoU. Instead, regardless of the fact that Tony accidentally created a rampaging murderbot and physically traded blows with Cap when he went ahead bringing Vision to life, the two make up as if nothing had happened. They stroll across the grounds of the New Avengers facility, palling around with Thor, Stark joking about retiring to a farm with Pepper. It's a huge missed opportunity to put in place animosity between the two; when Civil War rolls around, their beef has to begin anew, when really the Brothers Russo could have had a much easier time setting up their conflict had Feige, Whedon et al left the pair in a bad place at the end of Phase Two. I wouldn't be surprised if they regret it. What else should Marvel regret about the MCU? Share your thoughts below in the comments thread.