Marvel Heroes, apparently the team and the community felt as though the team-up system needed more work. That's why Gazillion has a major revamp of team-ups planned for Q1 2015.
According to a new dev diary on the subject, the revamp will cover a range of improvements to the system. Team-up characters will see more customization, expanded power trees, a level cap of 60, increased survivability, synergy bonuses, and new powers on top of improvements to old powers. Players will also gain more control over what their team-up companions do on the battlefield.
In addition to the developer diary, there's also a forum FAQ to deliver even more information on the planned revamp.