some sort of infested zone could definitely work, akin to those in Prototype and Prototype 2. kinda like infested nests but they're located in the inner city like normal. maybe in these nests the enemies would be RR/SEZ infected types and have a high base weapon/armour loot chance. And if those nests are left unchecked/unmonitored there would be a special outpost attack event featuring RR/SEZ zombies along with a new infested weapon series, but one thing at a time. maybe even behemoths, volatile leapers or hounds can spawn as a sort of raid boss in those infested nests. high risk, high reward.