Capture the Flag? Death Match, Team and single? Double elimination tourneys? King of the Hill and Turret defense? Gladiators? All of the above?
While Death Match and Gladiators were certainly fun, CoH didn't have many other arena options. I'd love to see all sorts of arena styles, so my answer would be "All of the above, plus probably the other stuff you guys think of". I'd like to see Arenas be more than just PvP matches, too. Races, concert, conventions and sporting events all happen at Arenas. Bookies that can actually take bets would be amazing.
Note: I'm not talking about Open World PvP or Base Raids. I like the idea, and I support OWPvP and Base Raids, but this specific thread is for Arena Games.
Second Note: I am well aware this is probably far outside the scope of launch-day material. At best, this is a year down the road, minor expansion stuff. Doesn't mean we can't dream.