Any sim that includes the ability to walk around a station or planetary base when you land on it would be awesome, as well as seamless transitions between space and atmosphere. Have a system that will guide you into the base you're going to land at and a landing autopilot or something if you don't want to actually have to land your ship. The next gen possibilities for space sims are pretty endless, and yet most just pump out the standard privateer/freelancer type deal. A sim in the vein of elite without the crazy newtonian physics would definately be cool.
It's why i'm eagerly awaiting an english translation of The Tommorrow War.