This is more of a idea for space marine 3. If it could be implemented in 2 that be fantastic. A 12v12 mode where the space marines are the attackers the objective is to make sure that the imperial guard reach a warp gate to destroy. Sub objectives would be destory smaller warp gates. And clear landing zone for more imperial guard to land. 

The chaos objective would be make sure they secure areas for warp gates for more infrentry reinforcement to make it harder for the attackers. The sub objects would be to fortify the landing zones the space marines are trying to take. The attackers lose if they lose all the imperial guard and there tanks. The defenders lose if there main warp gate is destroyed.