Multi-player battles would be fun.
Option during duels to click a toggle that offers the other player 'best 2 out of 3' then you play each other again
in game chat log (ie kongregate style)
maybe a combatant defined wager system, or alternately, several more PVP options with set wagers (100, 250, 500, 1k...) that would make you sit on the edge of your seat during battles, eh?
oh yes, and I personally think it would be great on the leaderboards if 'week' and 'month' score counters were reset every cycle. have the score open up with all time, but give an accurate portrayal of who is kicking butt (particularly pvp) currently. hell, even a way to submit a challenge would be cool