I don't think a horde mode is the right idea for this game, but my idea is to have a large skyscraper added to the game. This tower could have been on lockdown and something went wrong. The citizens couldn't get out after some turned. Now the tower is level after level of crazy challenges and requirements. Loot increases with each successful floor completed. Difficulty increases with each player. Side missions outside allow access to higher floors with the objective to understand what happened. Tho could be a directive given to the player by the GRE.