1.) A penalty for crossing the Pit Lane exit line.
2.) Stop go/drive through penalties for jump starts, passing under caution, dangerous driving and the rest.
3.) Slippery grass instead of the current 'bouncy' grass, wheels loose traction on the grass.
4.) Realistic damage sensitivity, for example, 99% of the time you touch someone's rear, the front wing will be torn from your car.
5.) Tyre barriers engulf the cars, instead of rebounding them away.
6.) Cars don't roll like pieces of paper (rocking back and forth upside down in GP3 looked bad), the videos look like this is already corrected.
7.) Warm up lap, which you control and then have to manually stop on your grid position.
8.) Catchment fences on the tracks. Would also make some crashes look more realistic, because in GP3 some of the cars would fly up and hit the invisible barrier above the wall.
9.) Oil leaks result in a streak of visable oil following the car that it occured to, and would be slippery to drive on for 3 or so laps, yellow flags would remain out if there is no safety car, which brings me to my final point.....
10.)
SAFETY CAR!