Just had the idea!
The oldest, Capture the Flag game. All arenas are like public dungeons (not instanced) except with teleport option. Each arena could be tied to a theme or a zone, e.g. instead of special maps, just pick an random area from pve zone every time; could be plain field, mountains or towns, as long as there is no quest there.
Two teams, NOT 3. So it's simple and clear to everyone. No alliance, anyone can join or leave in the middle, fast respawn, teams are always auto-balanced during the game.
No premade group and non-participants could watch either by entering the arena as visitor or from outside.
Like huge, public battlegrounds. Would be nice yes?