I'm sure this has been suggested before, so apologies if it's redundant. I was thinking that a CTF mode addition to Arena would be amazing. Two teams of 6 players each start with 2 sloops at your fort, maybe Hidden Springs for one team, Kraken Watchtower for the other team. The "flag" is actually an Arena chest, which rests on the pedestal in each teams (open) vault. The game plays like traditional CTF: the goal is to steal and then touch the enemies flag to your (returned/home) flag as many times as possible in the time limit. Your flag can be picked up in the field by the enemy, or returned by you to your home pedestal. Most captures wins. There are so many ways to play this: all out naval assault, full island defense, or any number of combinations.
Like I said, I'm sure variants of this have been suggested, and Rare has already indicated they have no interest in updating Arena anyway, but it's fun to think about what could have been. An Arena with CTF, King of the Hill, TDM, Last Ship Standing, and other variants might have really taken off.
