What a lot of people seem to forget about the Civil War storyline is that villains were also very prominent in it, as they were asked to register as superhuman just like the heroes. Moreover, some of them did actually sign up. Therefore, don't be surprised to see a few turning up in the movie. Some will see it as an opportunity to right their wrongs, while some may play along in order to lure the authorities in to a false sense of security or be released from detainment. Of course, there will be plenty of villains who aren't interested in registering and, as a result, there may be villains fighting alongside the heroes on both sides. Abomination, Absorbing Man, Brock Rumlow and many more characters could be involved - and that would be awesome.
