I actually like this suggestion, if it's handled properly. So long as the areas are small and not, say, a full zone that people who don't want to PVP would be restricted from entering, why not? In fact it reminds me of Lineage 2, which while having possible open-world PVP, heavily discouraged it with its karma system. So there were small "arenas" where you could enter and PVP freely, fenced-in areas which allowed spectators to watch from outside. Crossing the threshold of that area enabled it, leaving again disabled it, nice and restricted so that nobody felt forced just because they were passing through. Something small-scale like that could be fun.