3) Thunderbolts
With DC bound to have a hit on their hands with Suicide Squad, Marvel should introduce their own team of reformed supervillains: the Thunderbolts.
The Thunderbolts team was initially introduced into the comics with a fantastic bit of misdirection; they at first seemed to be just your average bunch of heroes before it was revealed they were actually Baron Zemo and his Masters of Evil in disguise. Seeing as Zemo will be introduced in Civil War, perhaps something like this could be pulled off in the films.
Alternatively, another incarnation of the team (which included Ghost Rider, Agent Venom and Red Hulk) were a group of anti-heroes, aiming to make up for their past sins. If Marvel went with this option, it would be a nice counterpart to Suicide Squad, which will feature villains made to do good against their will.