This could be implemented like a launch option where you choose the opposition (perhaps using a shared number or pass phrase) and then wait for your opponent to launch. Two people (or teams) with the same number or pass phrase will drop against each other. Otherwise the match search will time out.
To account for the server resources this requires, add a MC cost to such an arena. To make it really interesting, make the loser pay for the match. For example, pay 1 MC per person for a 1v1 duel, with the winner getting 2 MC (or 1 MC, so the house gets a cut and this system can't be used to transfer mc). This way there are stakes to the match and people can put their cash where their ego is. Perhaps allow the designation of the MC price, so people can decide how much to wager on their match. Let the house take a cut of the winnings to pay for the server resources.
This can generalize to team vs team matches, so clan scrimmages or matches can also have some financial stakes.
Edited by RedMercury, 13 January 2013 - 09:14 PM.