My 3d engine is a pretty good structure around which to build a game, but what game to build?  I think the best candidate I have is the sci-fi series I have been developing.  Perhaps you could start out as some sort of soldier on board the station, plot, space missions, aliens, giant guns, sex later, you command your station as it splits from the main.  It's a great infrastructure for a plot I think, but I don't know what that plot is.  People are begging for 3d space combat though, myself among them.  Building and maintaining a space station presents its own gameplay.