1) Fix charactermovement, damagecalculation and solidify objects so there's no more clipping or bouncing health. Rework all physics to include stray bullets that deal damage, instead of just bouncing off whatever they hit.
2) Rework and optimize maps, so that the number of zonelines can be reduced.
3) Rework the whole combat system from scratch, remove the target-boxes and give models a complete framework of damage-zones, just like every FPS today.
4) Content, fix the broken classes, add new things to fill the gaps.
5) Remodel the current player models to actually look like people, instead of the current anorexic anime characters.
6) Update the old world textures, all of them, to fit the new shiny playermodels.
7) Fix the mission system in the Citycom. Currently, getting a mission's like a box of chocolates. You never know what you're going to get. And when you're out of your favorites, you go postal.
Extreme? Yes. Necessary?