Disclaimer: I'm little more than a complete newbie, so I'm aware that my proposal may sound ridiculous to experienced players. I've just unlocked all science in the carreer mode without even going beyond the orbit of Duna or landing on another planet. It seems to me that a new tier of extremely expensive science could add motivation for further exploration. Each technology from that tier should cost realy much, e. g. 3000 science, and unlock a superior piece of equipment that can make exploration of the outer planets and Jool moons substantialy easier. Also, the combined cost of this techs should exceed the maximum accumulated science, so that the player will have to make decisions about what to research instead of unlocking everything after a few Mun and Minmus missions. Example tech: advanced nuclear propulsion, requires nuclear propulsion and meta-materials, unlocks an advanced nuclear engine that has 2x mass and fuel consumption of LV-N, but 4x thrust.