The multiplayer is only accessed through the single player. For someone to play in multiplayer,he would have to travel to the city where the arena is located.
When the player presses the key to enter the arena,he activates multiplayer mode,and the game automatically searches the internet to look for another player.
The game chooses a player who is also in the arena randomly,and both players engage in an arena battle.
Of course it will filter through players,to match players based on options.
All gameplay from the single player is available to the multiplayer too. Players can use swords,magic,potions,power attacks etc.
That way the multiplayer could integrate in the single player seamlessly, without it getting annoying,it would fit the lore and whatever the story of the main quest is,and there wouldn't be need for different gameplay mechanics or level design,so it would only need small work for it to work,so it wouldn't svck time and resources from the single player.
What do you think ? Do you like that idea ?