Build a arena on the Last Millenium world (or other) with a small waiting room (for the players that take some time to load dimensions). When you are on "lobby" you don't have active pvp, but when you fall to arena, pvp is automatically enabled.
An arena will be great, but for what modpack? And also are you daying this to an especific person or everybody can try it?
i would love an arena in all servers as its fun to pvp but to have an actual pvp arena would be sweet instead of just going away from claims to pvp