Daily mission.

Basically, there could be a small system added, that can work well. So, maybe there could be these little "missions" to complete. They would change every 24 hours, once completed, you would have to wait for next one. Missions would be made for everyone, adapting to their level, rank and weaponry. So, there would also be these donor missions.

Spoiler: Prize

Prizes could be: certain amount of exp and credits. They will change depending on the difficulty.

Spoiler: Missions - Levels 1-10

1. Get a kill while sneaking.
2. Get a kill while jumping.
3. Kill 2 enemies within 10 seconds
4. Get a kill with your secondary (pistol)
5. Reach 5 killstreak
6. Kill an enemy with your knife.
7. Survive a round of infected gamemode.
8. Capture a flag

Spoiler: Missions - Level 10-20

1. Kill an enemy from 20 blocks away.
2. Get 3+ kills in 9 seconds.
3. Get a 10+ killstreak
4. Launch an EMP.
5. Kill 3 enemies with your secondary.
6. Win a FFA
7. Vote for a grenade
8. EMP as a zombie in infected gamemode.

Spoiler: Missions - Levels 20-30

1. Launch a tactical nuke. (21 killstreak)
2. Kill an enemy from 45 blocks away.
3. Get 6 kills with your secondary.
4. Get 30+ kills in one round.
5. Kill 2 enemies while jumping.
6. Kill 4 people in 8 seconds.
7. Get a 18 killstreak.
8. Kill a player who is carrying your team's flag.

Missions would only last till level 30, since they would boost players towards faster prestiging. Now to the donor missions. They would appear if you have a certain gun.

Spoiler: Missions - Donators

Minigun:
1) Get 20 kills in one round with minigun.
2) Launch an EMP with the minifun

LAW:
1) Get 10 killstreak with LAW.
2) Take down a player from 20 blocks away while using LAW.

Ray Gun:
1) Get a collateral with Raygun (2+ kills in 1 shot).
2) Launch an EMP with. a Raygun. (Infected counts).

Flamethrower:
1) Burn 5 enemies in 1 life.
2) Get a total of 10 kills+ with a flamethrower in 1 round.

Crossbow:
1) Kill an enemy from 40+ blocks away.
2) Kill an infected player with a c-bow.

Nades:
1) Get 3+ kills with a single grenade.
2) Blow up 5+ survivors/zombies in infect.

Flashbangs:
1) Stun 2 enemies with a single flashbang.
2) Stun a survivor in infect.

Note: Donor challenges would be weekly, and would only apply to one purchased item.

 

They count in for donor missions. Remember, disliking will not affect my work
Also, why would you need free stuff anyway. For instance, look at all the donors out there. They have challenges/missions, too. After all, they could be added with a prestige system.