Add more power cycles, nozzle types and more fuel types. Also add in a feature where engine plumes have a direct effect on the surroundings such as being able to heat up and push away what is directly in front of the engine nozzle. If possible, make engine and rcs plumes respond to surroundings, for example, just before sn8 bellyflops, it ignites its engines, the plume can be observed to be bent a little due to the air resistance, another example would be falcon9 during entry burn, the plume is blown back towards the rocket due to its high speeds, this can also be applied to rcs thrusters as the current plume is plain and undetailed.
