I'd suggest star control 2. The main campaign is massive and the design was way ahead of it's time (long term reprecussions to your actions, sandbox gameplay rather than railroading, a universe in which things will happen with or without you, etc). On top of that the scenario mode isbasically a coolection of short stories, and I don't know how many hours I spent in melee mode (roster based pvp, you each pick 5 ships)