Okay, my idea is a mode where a group of players either defend or attack a fortress held by Garleans, beast tribes, etc.

In the defense section: a group of players (8-16) are placed within a fortress and are expected to hold off wave after wave of enemies, similar to the Firefight mode in Halo games.

You are able to leave the fortress, but if every player leaves it, or all the players in it die, it is taken over. If the players simply leave, you are able to reclaim the fortress. If all players die, however, the battle ends.

What can be done when you leave? You can sabotage gear, steal supplies, and other ways to hamper the enemy's invasion

In the siege: In Total War-style, a large group of players (again, 8-16, maybe more (for both Defense and Siege)) raid a fortress held by the aforementioned enemy factions. Note: The aforementioned sabotaging does work for this. (If it's possible, anyway.)

Thoughts?