I really would like to see more life on Kerbin, maybe some cities that we could see the lights from space, as it's just a giant greenland. Also some biological life to discover on other planets, maybe another kerbin like civilization too. Instead of just the kerbol system we could have an entire galaxy. But what i find missing the most is for exploration, mainly planetary features when you land like: erupting volcanos you could try to land close, sleeping volcanos that would suddenly erupt, or sudden ground breaking earthquakes at your colony site, dust storms/tornados, tsunamis and all sort of different dangers other planets have. Maybe even have a weird biological wormtype monster who could swallow your ship.. The list is infinite with the possibilities of the universe. Ofcourse we would have new science tools to prevent and detect all these scenarios. But my main concern is that all the planets we currently have and land on are all safe and there is no real danger making it a bit boring.. You can't really land on the "wrong" place unless you land on a slope and your lander tips over..