Guild Wars 2

I really want something larger than 5v5 but still with balanced group fights.

10 vs 10
&
15 vs 15

Could be very interesting especially if the players can build the maps they fight in.

They didn’t specify any limit. Though, as there can be up to 4 teams (if I recall) in the arena at any time, one can assume that each team can be at 5, perhaps even 10 players or more.