One of the things i missed most in KSP were realistic sounds in atmosphere that make you shiver.
One example of something like that is in the link below.
KSP didnt need that because you wouldnt just hear another aircraft passing you since there was no multiplayer. Imagine a helicopter flying above you. you hear that even miles and i think that is one of the things that make you appreciate the raw power those machines have. Rockets, Jets (especially when breaking the barrier of sound), rockets and helicopters come to mind...
another suggestion for engine sound would be screeching engines (think tie fighters).
as far as i'm concerned this sound often indicates a pressure error but this would still be a nice addition
If you've ever flown in a plane you probably know what an incredible sight it is to fly above clouds and see the sun set while its storming below the clouds. I don't know how far you will go with the weather and its effects on visibility but things like these really play a big factor in Immersion and fun. we play KSP for the fun of it but i always missed the visual sensation when not playing with dozens of mods. not the graphics alone make a game great, the visuals and sound design set the mood