2. Magneto
While Magneto might not be the most popular villain in Marvel and DC Comics, the fact is that Ian McKellen's portrayal has been the stand-out aspect of every movie he's been in - and Michael Fassbender's performance as the younger incarnation wasn't too shabby either. Magneto has such an interesting back-story that it would be an absolute crime not to explore it fully on screen. Although X-Men: First Class did this to an extent, the full experience of his childhood would be great to see up until the point he met Charles Xavier for the first time. A specific, titular origins movie for Magneto has long been touted - the origins concept being something that the X-Men movie franchise isn't unfamiliar with, having already produced a Wolverine origin movie - and this isn't something that should be overlooked for much longer. He was born sometime in the late 1920s into a middle class German Jewish family whose father, Jakob Eisenhardt, was a highly decorated World War I veteran. They survived terrible discrimination and hardship during the Nazi rise to power, Kristallnacht and the passing of the Nuremberg Laws and fled to Poland in 1939 where they were captured during the German invasion and sent to the Warsaw Ghetto. It was these terrible beginnings that would lead Magneto on to the path he ultimately followed. Snippets have been shown of young Magneto in Nazi prisoner camps and, of course, the scene in which he accidentally killed his Mother was one of those scenes, but the rest of his childhood and adolescence would be what ultimately shaped him. Growing up dealing with these events, as well as coming to terms with his incredible mutant powers, would be something that would allow us to sympathise with the character some more, as well as something that would be a very interesting story.