A similar idea of adding a duel script for those who want to have a private duel has been suggested before, however I'm not quite sure if something like that will be added to the gamemode. As for the "Or do /arena always open" suggestion I'm afraid it won't happen, since the Arena was orginally created for temporary events, and not to be opened forever. Otherwise the game will get boring since it's alot easier to make kills, and find an opponent to duel within a short period of time when it's open. Everyone will be just inclined to teleport to the arena as it takes less effort to find an opponent than when you're searching normally. I hope you do get my point.