It dosen't need anything especially Zombie mode. If you need a Zombie mode, it needs offline bots yes, but Zombie mode? No. Can you add dogs? So the players have to think of looking down for dogs and looking for other soldiers. Don't add any supernatural people please! It's gonna hurt the rep. Just add some more details maybe you can make it so you can creat your soldier, skin colour and such. Like if you agree.
I totally agree with NukaSixty2. Keep it the way it is, but it wont hurt to add more detail and secret hiding spots. Maybe all you need is more detail. Im not saying you SHOULD do it, just suggesting.
First, I like what you did adding the new levels nice touch. More levels would be nice. Also you should be able to be more than Marines and Insurgents. Maybe Chinesse and Russians, or different terrorist. Otherwise nice game I like the ranks and guns and everything you added, it's great dont ruin it by adding memberships or things were you actually have to play, leave it the way it is it's awesome!