Adding swimmable and non swimmable water, or adding water level detection to make the character walk over puddles instead of swimming would be nice.
Enemies with different combat behavior and skill. For example make an enemy move around but very little (or absolutely not move) but also attack enemies instead of being totally motionless, or modifying the reaction time and accuracy of a single person, so you can make more difficult enemies without relying only on weapons and health. That should give a really good variety since most boss fights and stronger enemies in custom maps are basically bullet sponges or carry around modded guns instead of being just better at shooting or dodging.
Also, about the skin editor, saving presets would be very nice.