Many others versions of Ro have an open PVP servers. I think having PVP would add so much more depth to the game, especialy in combination with the War of Emperium. I do know that there has been many requests for a PK server, but to my surprise, both USA Ragnarok Online and EU - Ragnarok Online have ignored these request.
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Currently not enough players. They would have to merge Loki and Iris, then make Iris PVP. The Merging Software is still in its infancy stages for Ragnarok Online. pRO is getting a couple server merges this May, but it was pushed back because the software from Gravity isn't done yet. Also you have to consider that PVP in a game not based on Full-PVP would be unpopular as it has in the passed, with only 10k-50k subscribers, it would have a very small audience to warrant a server unless they merge loki and iris.