Kerbal Space Program

It's fine that our Kerbals don't need food and have an unlimited supply of cabin air or even in their EVA suits, but I think the game could be given an interesting twist if we make Kerbals retire after 30 years of service. If they are home, they simply get removed from the available crew. If they are away, they become tourists and you are automatically given a mission to return them home within 5 years. This would be like an HR contract where killing the Kerbal, or otherwise failing to get him back, will result in loss of reputation and money (paid restitution to his family) but getting him back could earn you favor.

This would also be a good step moving forward for another expansion pack to explore more planetary systems, requiring cryo-beds that prolong their life and career time by possibly slowing it down by 40 times (or more) while in the cryo-bed. The expansion could include some colonization features to help you build a new base fabricating materials and make space ships in the new system. This would also spawn a whole new kind of tourism, colonists. These Kerbals would be handled like tourists in their lack of usefulness on ships, only you don't have to get them back to Kerbin (until they become home sick and hire you to take them back).