A character whose backstory I think is screaming to be fleshed out in a One-Shot is Howard Stark. How did he go from the likeable guy we saw in Captain America: The First Avenger to the cold and distant man Tony remembered him to be?
Obviously, recreating the Tessaract particle he studied was very important to him or he wouldn't have hidden the particle design in the layout of the Stark Expo for Tony to discover years later. Why was it so important to him? Was he really the "I prefer the weapon you only have to fire once" guy described in Iron Man or did he have a larger agenda? Such a story could show early interactions with Obidiah Stane and Ivan Vanko.