I'd love to see a system where Kerbals have a life cycle to where if a vessel is labelled as a colony, then slowly more Kerbals may inhabit it as an effect of reproduction. Another Great feature I'd love to see is not only weather such as rain or snow or storms for Kerbin which affect the conditions of launching a vessel (such as wind), but those conditions may only be for a certain region and launching from another location may let you have better conditions. (Another awesome thing for Kerbin would be seasons that would make the grass different colors such as white with snow during winter, and the rain being replaced with snow during winter.) There could be weather on other planets, such as salty rain on Laythe or Dust storms on Duna (Possibly if a dust storm occurs, we could see the efficiency of solar panels and the accumulated dust on them by right-clicking. Having an engineer have to clean them would be realistic.) Having these weather effects affect things such as taking tests with the atmosphere or using a thermometer could be a feature for showing off how amazing KSP can be with these features. Other things they could affect would be solar panels due to sun exposure from the clouds. Anyways, I understand this would be a huge thing to put on the devs, and that's why I'd still be happy even if it was a 30$ DLC. I don't expect many of these features (such as the life cycle thing) to be implemented, be we can always hope.