Overhauling the weapon system and combat mechanics entirely, adding more variables, customisation options, etc. To completely turn a gun around. Most importantly to, An administrative panel within Dev server environments where changing the weapons stats would be done conveniently rather than through code digging, and presets for different tickrates to keep performance consistent during tickrate changes. There wouldn't be set meta attachments like now, Each gun would benefit differently from whatever attachment is on it. Stuff like movement speed on the M82 and AS50 were steps in the right direction, I think movement speed changes should be applied to several guns, With lighter guns having better movement speeds than others at the tradeoffs of less recoil and range than larger weapons.
On the topic of weapons too, Modernising existing weapons and adding new ones that would be expected in the 21st century. (Give me my TTI Pit viper and SIG MCX)