"The only way they could really do the story justice, is to have three films, followed by The Agents of S.H.I.E.L.D. becoming the actual TV adaptation...this spanning 5 seasons!"
Lo and behold this intended act causes turmoil and a growing rift in the super powered world. Secret identities are secret for a reason and exposing the capabilities and weakness of their powers is strategically, not in their best interest. Eventually those who are against the sanctioning of this act and those in agreement, face off against each other - a massive battle ensues amongst them hence the title Civil War.
The Civil War narrative is massive, I mean 70 issues massive, with the story, crossing over into different universes and with sub-plots galore. The only way they could really do the story justice, is to have three films, followed by The Agents of S.H.I.E.L.D. becoming the actual TV adaptation...spanning 5 seasons!
Also, key to the proceedings is what has to happen before the Civil War edict is implemented. I would guess that a second bout of high civilian casualties and mass destruction caused by yet another conflict, such as the events of Avengers Assemble is a good enough reason. But I bet you it is in the events in The Age of Ultron that will lead to the supreme conflict.