The most important element set in motion for Civil War is that Captain America now appears to have his side of the war completely set-up, what with Cap and Black Widow commanding a team consisting of new recruits War Machine, Scarlet Witch, The Vision, and Falcon. Though others could easily join them, this is already a pretty hefty number of powerful enemies for Tony Stark and co. to take on, and that's without much of a well-formed team of their own. Though allegiances could change a little bit once the antagonism ramps up (especially considering Rhodey's loyalty to Stark), it would make sense for the "new Avengers" to form one side, and Stark's own force to form the other.