Basically, it is a persistent 8 vs. 8 PvP match, with midmatch joining and leaving. Steady stream of people coming and going midmatch, never know what to expect, but you can drop out at any time, change build/character, and come back in if "that one griefer dude running away/tanking/whatever" is getting on your nerves.
Rules:
20 Faction to the whole team per kill
Characters are kicked out after having been dead for a set period of time, varied based on the number of people waiting to enter. (From 30 down to 10)
If no one is waiting, players are simply resurrected at the spawn point after 30 seconds.
Players spawn in a separated section from the action, where they get 20 seconds to prepare before being teleported to their spawn point.
NO title from the arena
After a set amount of time (10 minutes, perhaps?), a player may be kicked from the match if there is a waiting list of a certain length.
Couple choices I can think of as to how the match would work:
Standard GvG- Generic GvG match, requires less programming, but not balanced for disappearing and reappearing people.
Generic death match- Kill people and get faction until you get killed yourself. Could use any suitable map.
Something else- relic runs, point captures, whatever suits your fancy; may have to modify the rules for some of these, though.
Another decision:
Choose which of the available arenas to join? Or be put in one randomly?
Choose which team side you're on? Or be given one?
Bring a few teammates? (Up to 4) Or just stay full random?
Additionally, as a way to cut down on long waiting times, if they were to occur, there could be alliance specific arenas of this variety. Only people from the alliance that owned it could enter a match there.
Why? Casual PvP for fun and faction, no title to draw in too many farmers, and easy access PvP that is 8v8, a nice thing when the skills for PvP are balanced with 8v8 in mind, yes?
S

Not connecting any title to it is also great, some faction is enough.