Several problems. 1. FF is not a pvp game traditionally. 2. Lack of experience of the developers with pvp(probably). The third and biggest issue, "balance". It would take a lot of time, effort and energy to get things balanced. Every spell and skill would have to be re-evaluated to see what effect it would have in pvp. A lot of them would have to be altered, changing the damage, duration and effect.

It is possible, but it requires a lot of time to make it right. Time that I believe the developers would be better off investing in something else. They are in a race against to clock to make this game work, make it fun, keep people playing and attract new people. And in my opinion a simple pvp-function would not be a big enough feature to make a difference in that.