If anyone has ever heard of Outer Wilds, my wish for KSP 2 (which was actually my wish for KSP 1) will sound familiar to you.
I would like something similar to what we have now; Lego meets Orbiter meets Buzz Aldrin's Race Into Space. I love that aspect of the game. I would, however, radically change Career mode, to the point that it might turn off a lot of people. Please bare with me. I like me some Sandbox gaming, but honestly, a scripted story with set goals can be very fun too. I wish for a KSP "Story" mode would actually give missions that need to be accomplished to progress an overarching story... that is the discovery of a Stargate like thing at the edge of space!
The campaign should be roughly aligned with the discovery of new technology. At first, you are doing simple things like reaching orbit, putting satelites, etc. But, as you start exploring the planets, you can really unlock the secrets of those planets. This is where the idea of Outer Wilds comes in. It's sandbox, but there is still obviously something behind the scenes going on. As the career progresses you might find, say, a cave in Minmus. To enter the cave, you have to build a rover that allows you to move into the cave and explore it. Within it, you discover some ancient artifact or whatever that's a map to something on Duna. Doesn't have to be something like that, but it should lead you in that way. Obviously, at no point should you be stopped from diverging from the "story mode"; you can do as you please. In some ways, I want there to be soft "broken bridges" of sorts to encourage the players to continue the story mode... I mean, that's why you are playing story mode as opposed to the other modes, but it should in no way lock you into it. Kinda like what The Long Dark wants to do with it's story mode eventually.
I've never really developed the idea too much, but it's always been of interest to me.