Something I've wanted for years is having weapon mods being visible.
Having a bigger magazine if you use Extended Magazine 3. A scope while using hunting sigh 3, etc.
Would be amazing to see your weapon change a big when using different mods.
Expanding gameplay vertically would be cool too, adding some offide building where you can go inside and do missions there, get to the rooftops.
Have new districts, or expand the current ones with outside areas of the cities, with more mountain areas, making some vehicles better to drive in dirt.