I would like to add to all the people here,
1: Change the light of the V.A.B remove the shadows and put a more diffuse or global light because when I look under my constructions it's hard to distinguish the details. A light a bit like in ksp1 to see the details clearly.
2: Robotics of course, but with a few more specific features. For example, I'd like to have an altimeter so that as soon as you exceed a certain altitude, it triggers a kal controller or an action. Also a speedometer to automatically trigger actions at a certain speed. An anglometer that detects the inclination of craft, which can be handy for making your own SAS. Logic doors. I think this would develop almost infinite automation.
3: The rovers, and wheels in general. Make sure that when you brake a rover or a plane doesn't turn over as soon as you brake a little.
4: That you can see the remaining fuel in a single tank when you right-click on it.
5: Weather for colonies and crafts. For example, make it possible to have storms that delay the launch of certain ships, thus adding to the difficulty of controlling certain planes.
6: Radiation, to create radioactive zones later in science that would require protection for our little Kerbal.
7: More types of ore and different types of refinery to refuel our ships.
I've got more to say but I'll stop here. The most important to me are 1 and 2.
Thanks for this game, it's going to be incredible. See you soon ♥.