Solution: automated mass drivers. End game would consist of placing these mass drivers on different planets/moons, and they would extract specialized resources enabling you to build things like more mass drivers, etc. Fuel, ship building materials, etc, could be 'running out' on kerbin, caused by exponential population growth. But just a few mass drivers later...population is still expanding, and the race is still on. you need off-planet habitats, etc, until, one day, a genius working on distant Eloo comes up with the idea for a condom.