We all played GTA Vice City. If you remember, there were Kill Rampage Challenges in GTA VC.
Here is a Video for Reference.
My suggestion is to add some Rampage Challenges Around San Andreas.
Examples:
Rampage #1 (Destroy 10 boats/cars/helicopters in 2 minutes with a RPG)
Rampage #2 (Burn 30 gang members in 2 minutes with Molotovs)
Rampage #3 (Runover and kill 30 gang members in 2 minutes)
Rampage #4 (Kill 25 gang members in 2 minutes with a Combat Shotgun/Pump Action Shotgun)
Rampage #5 (Kill 10 gang members in 2 minutes with a Katana)
Rampage #6 (Kill 30 gang members in 2 minutes with a Chainsaw)
Rampage #7 (Kill 25 gang members in 2 minutes with a M4/AK-47)
Rampage #8 (Kill 20 gang members in 2 minutes with a Sniper)
Rampage #9 (Kill 25 gang members in 2 minutes with a Silence Pistol/Colt 45)
And Many More.
NPC's can be spawned as Gang Members for Rampage Challenge. Rampage Challenges should occur in a Virtual World.
Cash and Score Reward on completing challenge.
New Achievements set can be added for Rampage Challenges,
Doing Rampage challanges should also increase player weapon skills.
Pickup Object For Rampage Challenges