what I mean is that these "big" (can be from 10-50, i dont know) battles can be instanced, and the rally point is a caravan or something, which then leads you to the instance.
Sure it's not a sixy way of dealing with it but the big ass battle will be held privately without any innocent bystanders getting caught in it.
Also make the wars have max cap - smaller battles may only have max 10 on each side while the really big battles which is more rare will contain up to 50 or more on each side, i dont know
just throwing some ideas.
It doesn't have to be instanced but I think these bigger battles would suit better to be instanced. Gang wars and normal pvp should be in the "real" world.