Some mountains are sure made out of these molehills...

We've already said that SE can simply make it so you can opt out automatically from all duel requests. They could also just add a player status similar to Busy, where it shows that you are willing to duel other players. Being able to duel out in the open world would be fun as you would have all kinds of scenery at your disposal. Along with that, eventually the community will designate an area to meet up and just duel for fun, SE can even add a fighting ring out in the open field for this purpose.

Having players able to participate in the Colosseum would also be an option (the battlefield is already ingame) with gear caps to equalize gear and an restricted mode. Hell, SE can add some flare to it and players themselves could become famous Gladiators by participating in a tournament or monthly ranking in the Colosseum.