Although it may feel like every step of the MCU has been meticulously planned out, it's obvious from the early films that they were initially just making it up as they went along; a mock-up of Captain America's shield appears in Iron Man and Tony Stark's affiliation with S.H.I.E.L.D.'s Avengers Initiative flip flops from movie to movie (he's in, then he's out, then he's in again). The biggest of these, however, is The Leader. In The Incredible Hulk, Tim Blake Nelson played "Mr. Blue", a scientist trying to help cure Bruce Banner who's later revealed to be Samuel Sterns. The movie ended with him getting some of Bruce's irradiated blood in a head wound, pointing towards him becoming the Hulk baddie in the hoped-for sequel. And then... nothing. Like most of the plot elements from the MCU's black sheep (even Iron Man 2 gets treated with more respect) this was never referenced again. It's not really that much of a let down - while Leader would be a cool character to see, there's more enticing potential in the MCU out there - but it's poor form to just drop Sterns. If his role in the series really is done, a reference to an off screen conflict, or perhaps even a send-up of the character, becoming a S.H.I.E.L.D. prisoner before being completely forgotten, would tie up the loose thread nicely.
