So I'm not sure if this idea has been brought up before, but to add to the gun shops, I was thinking there could be something like gun upgrades. The general idea would be if you get a streak of 10 or more, the upgrade is activated. The upgrade would last until the player dies. This would not be a one time use upgrade. It can be used over and over.
Example 1: Streamer Upgrade (Fever Mode). When activated, the Streamer will do 2 more damage than it would in Frenzy Mode and have different colored bullets.
Example 2: Plasma Shotgun (Bullet Sprayer). When activated, the Plasma Shotgun will shoot 5 bullets instead of the regular 3 bullets.
Also, during the upgrade, the user's bounty goes up which would encourage more PKing in Skyworld. However, upgrades can only be installed for the everyday guns. (Nothing like Rudolph Rifle or anything like that).