Don`t we have the arena and the whole pw map for that?
-1 The pk and even rpk is pretty dead lately, no need to kill it more
We sometimes used the arena for guild pk events and never got botherd. Try that. Or origination map,
Not really possible to make entirely private instance for faction, like guild bases. But there many open, rarely visited areas that can suit well for guild pvp events, like Etherblade Outskirts, Courage Path, Vibrant Cliffs and many more.