Star trek game (Freeform or 2d20)
My idea is to do a mix of these things, yes there'll be conflict and diplomacy, but it is an unknown part of the universe, so we'll see spatial anomolies, new aliens and stranger things.
Another big part of the game, like in the series is the crew itself. Their lives, relationships and coping with being this far away from home, maybe forever.
I guess it would be a cross between TNG and voyager if it was broadcast on HBO.
As for crew leaving. Part of the RTJ will be that if you're away without notice for a certain time (14 days I'm thinking) you're considered AWOL. If you leave or are AWOL, your characters will be taken over by the GM team and suitably killed off if a replacement is found.
That being said, players are more than welcome to play junior officers or even enlisted crew.