COMBINE:
So i was thinking. in a few fallout or nuke rp type of game servers they allways have combine which attack zombies, you and antlions, like a 3rd enemy. its fun to challenge something that has a ranged weapon rather than just zombies and antlions, also adds to the rp factor as there will be combine coming to rid of anyone else incase there infected.
The combine could spawn around the map, about triple the rarity of the antlion mounds. so there not allways everywhere but on occasions they'll spawn on and keep an area clear of anything but them. its good having more a veriaty of enemies. The guns your probaly thinking would be a problem, but the way around it is to use the same code npc of the combine where when they die there guns burst into the particals and dissapear, so there wont be any drops. They could give a little more xp than zombies and antlions would as there harder.
QUESTS:
now i cant remember if you were adding some sort of quest system into it but i think it would be awesome if you did. it would add more reasons for noobs to stay, and it could be a better way of leveling up than just killing. quests like collectiong so much of scrap to give to the person and in returt they give you a medkit and 12 xp or something. give the server a wow factor.
Science:
become a scientist for the first time.
Reward: 1 small medkit (5 xp)
Make a total of 50 medkits
reward: 2 large medkits and 4 armour packs (10 xp)
Scavenger:
become a scavenger for the first time.
reward: 10 scrap and 10 small parts (5 xp)
scavenge a total of 100 scrap, 100 small parts and 100 chemicals
reward: 50 scrap and 50 small parts (10 xp)
and so on...
These are just ideas, and im sure there are either reasons why not to have them or that you are infact going to add them anyway. but i thought id share