As I see it, 8v8 can only be balanced around objective-based game modes. 4v4 arena is not too chaotic but having 2 tanks and 4 DPS would allow you to erase a player very, very quickly. I see 8v8 modes happening long before Frontlines is unveiled, maybe even as soon as 2.2.

There's not a lot known about Frontlines other than it is a persistent, PvP-enabled zone where all of the city-states/Grand Companies are pit against one another for map dominance (Realm vs. Realm). It's slated to come at a much later date - think 2.3 and beyond with all of the other stuff they promise but end up drip-feeding.