Guild Wars 2

Wouldnt it be a good idea to sell cards with unlimited use where the player and enemy can have an agreed duel and to outsiders, their toons turn grey and u can not interupt them or fight them ?

I know a few games already have this function

In WvW? No. It goes against the very principle of WvW – random chaotic war. Duels occur in war sure but its a function of two people meeting each other in open battle, not two people entering a closed arena. We generally honor duels (or GvGs) yes, but we could crash it. Thats what make it a true battlefield.

That said, I never understood why we cant do it in Heart Of The Mists waiting for queue or specific arenas in PvE cities. It would be great entertainment.