I saw a thread about adding Imperial as 4th faction. As much as it sounds fun, it will mess up current balance and require a huge rework of Cyrodiil.
How about allowing Vampire vs WW within same faction. If Vampire or WW value their racial heritage, they should be allowed to attack their eternal enemy. But if they do attack same faction, they will be flagged breaking military order and can be attacked by anyone without penalty. This can be on a longer timer than current guard mechanism. This should limit vampire vs WW to more isolated setting and not in the middle of factional siege or campaign activity.
This can lead to development of shadow vampire and WW organizations cross over to other faction. And maybe one day lead to mortals vs immortals conflict superimposed on faction vs faction.