From
http://www.elderscrollsonline.com/en-us - scroll down to the Alliances section, it lists the three standard factions, and a 4th - Imperial, and says the following:
As war ravages Cyrodiil, the Imperials have become fractured, seeking their own individual purposes amidst the turmoil. They can be found in the ranks of every alliance or opposed to all three, bringing their considerable skills to bear wherever they choose to fight.
Maybe I'm reading it wrong, but that last part sounds like Imperial is becoming its own 4th faction, that fights against all three of the others, or can swap between them all.
Thoughts?