Many little big things I would love to see in-game:
A) It could be difficult to implement, but, what about a replay system? Imagine something like this (something less complex obviously) https://apolloinrealtime.org/11/
B) Voice lines for the Kerbals in radio comms, no bootleg simlish please. Nereiuq is zov areñ im oczerfo, séver la onacixem loñapse ne nelbah Slabrek sol euq.
C) A photo mode, or even a Sandbox/director mode would be nice. I guess that will be on the shoulders of the modding comunity again.
D) Lightsails, useless gameplaywise, just for fun.
E) A kerbalized version of Elon Musk giving us a contract to launch a driveable roadster. You know you want this.