In my opinion a spectator mode (needs multiplayer) would really help newer players learn the game. They can watch the VAB / OAB building process, the testing, the actual missions, the accidents. Maybe add the possibility of slow motion replays (with the possibility to pause) where you can fly around with the camera and see everything that happens during a friends campaign. Maybe we could even exchange replays like save files. It's like having interactive YouTube / Twitch but you watch your friend play (and maybe talk / ask questions). Better yet, players could have the option to actually step in and interact with their friends if they wanted to control a kerbal during a mission or help design a craft.
PS: As long as spectating is limited to a volume around the craft and it's not used to discover easter eggs or secrets like in Minecraft.