What I would really like to see at this point is at least one other gas giant, preferably with a ring system and several moons. It should also be extremely far away, almost half Jool's distance from the Sun. It should be the final big destination, after you've visited everywhere else. Aside from new planets, I'd really like to see improved terrain. Way more craters and tall mountains on Duna, maybe even a large valley somewhere on the planet. It's these things that make the real planets cool, it would be awesome to explore things like them in KSP. It would give a reason to build bigger and better rovers that can traverse complex terrain too. A game mechanic I would love to see is things that can affect the duration of missions. For example, decay on RTGs, which could be realistic - when we time warp in KSP it is possible to skip forward quite a few years, some of my missions are up to 7 years or more. That's more than enough time for these power sources to become less efficient. Another thing I'd really love is dust and other elements on certain planets (such as Duna) that could effect solar panels. This could tie in with weather (which is also an important thing in my opinion), which could clean off the solar panels periodically, but not very often. If you didn't get a weather event for a while and your solar panels got too dusty, that would be the end of the mission. If you had kerbals nearby, or it was a manned mission, they could clean the solar panels too (much like you repack chutes). I'm not expecting any of this in 0.22, or even 0.23, but I think some of it would really boost the realism and benefit the game. Especially things that can shorten the life of missions, thus providing reasons to build new ones every so often, and maintaining the ones you've got. That's a game mechanic that would be really nice in my opinion.