The ability to suspend play at any time. A pause menu option that quits the game but takes you back to the main menu. You can resume the game from that point but only once. This restriction stops you turning it into a quicksave system, but still allows you to exit whenever life demands it. It would probably be in addition to some checkpointing system.

A fair number of games already do this, but I can't think of any examples off the top of my head!