Nope.

Not a PVP Game, so no open world PvP, this includes dueling. Open World PvP (that isn't in some form of instance) is contrary to the cooperative spirit of the Final Fantasy series.

Even if they were dueling made available, it would be under similar restriction as Tetra Master due to the same reasons, location wise. This is to control Duel-offer abuse and provide reason to go to the designated areas.

Class Balancing is non-existent, for PvP, which is why even in Slaughter Mode there are objectives to fight over. PvP here is a team-game. Even dedicated PvPers recognize how little depth and weight it would add to the game vrs creating better PvP matchmaking and better modes than we currently have.