Or make specific zones where this can happen.
DAoC, Warhammer, and several other games do that. You have the "Frontier" zones where there is PvP and PvP objectives, and then the rest of the world where there is no PvP.
FFXIV would be a lot like the DAoC servers that were "co-op" where all three realms could go anywhere, join any guild etc... but out in the frontier it was Guild vs Guild, or realm vs realm. Those who didn not wish to participate could simple just no go to the PvP area and there was tons of stuff for them to do.
I think an addition of an zone or two that is like that would be awesome.
