Call it Battlefield or something like that.

This will be more dangerous and challenging than the Arena.  Because on the Battlefield, you have a chance of losing silver and/or a random piece of gear to the other team when you lose.  But you can only lose gear that isn't equipped or locked away safely in your Armory.  But your can Armory can only store a certain amount of items and must be upgraded with silver or gems in order to store more gear.  And your silver can also be stored in a Bank.  But again, the Bank can only store a certain amount of silver and must be upgraded with silver or gems.  Oh, and you only have a chance of losing silver or gear when you attack, not when you defend. 

If you have a Rogue Champion in your party, then your chance of losing silver or items is decreased.  Just as your chance of winning silver or items is increased if you have a rogue on your defensive team.  I started another thread about introducing Rogue champions not too long ago, so you can look that up if you feel like it.

What actually makes the increased risk on the Battlefield worthwhile is that you can earn twice the amount of medals as you can in the Arena. 

EDIT:  Almost forgot.  Your team/group must be of the same Alliance (Telerians,Gaellen Pact, the Corrupted, or Nyresan Union) or they get penalties on offense and defense.  And if your team is all of the same Faction, then they gain bonuses against certain other Factions.  For example, Dwarves gain a bonus against Orcs and Ogryn Tribes, Orcs gain a bonus against High Elves and Banner Lords or Barbarians, etc.