It occurred to me the other day that the upcoming justice system might be the perfect vehicle for something many people have been asking for since launch. Take a short leap with me here for a moment. What if... there was a way to change alliances via the justice system? Would you want to try or do you know someone who might?
For the sake of a degree of realism or immersion, it should not be a simple choice or task. It should actually take real time and effort on a players part to alienate his original alliance while trying to gain the trust of a different one. For this reason, it would be necessary to finish the quest lines for all three alliances to open the door, so to speak, and then by shear determination and effort become "public enemy number one" in the alliances to which they do not wish to belong. To do this they would need to raise their bounty to a high enough level in all the major cities in two of the three alliances. If done successfully, they could then begin courting the remaining alliance.
At this point there should be a quest line. Once a player has gained the appropriate level of infamy in two alliances they could, for example, act as a spy, assassin, or even a privateer for the alliance they want to join. This quest line should be extensive so it actually feels as though something has been accomplished in the end. Considering what the player would be trying to achieve it would also be appropriate to require that the player not be involved in any PVP in Cyradiil throughout the entire process (or at least after achieving a high enough bounty in one faction). How could you ever gain the trust of an opposing alliance while actively participating in the the war effort with their enemy?