This is a few suggestions I would like to see in the game;
Add the option to remove bullet casings... Do we really need to see bullet casings come out of the side of the gun. The reason I would love an option to remove this is because we I shoot to many shots at say a super zombie to fast I lag out and freeze... which doesn't really help me or you could even do it so they disappear faster or only a certain number of bullet casings can only be around at anyone time.
Then something that would help people with lower end computers but I don't really mind if it's not added is the ability to enable or disable seeing dead zombies. When to many zombies have died around me my game freezes a lot more or same as the bullet casings remove them make them de spawn a little bit faster or only allow certain amount of zombie bodies on the group at once.
Make it so we can see clan members on the map. - Like we can with groups
These are just a couple of suggestions I would like to see added and the top 2 are mainly just for performance purposes so more people on lower end computers can have better fps while killing zombies or pvping.