Regarding the storyline question, yes, you can create your own scenarios with predetermined events, but it's a painful and time-consuming task, since there isn't actually a scenario editor - you have to set up the initial conditions by playing out all the empires and getting them in just the right positions!
So there aren't very many scenarios available for SE4.
If you're looking for storyline, though, you can always join a PBW game, lots of roleplaying there, as Grandpa Kim said - or wait till summer when Starfury is released! (Think Privateer, Freelancer, or Elite, only with SE4 ships and races!
)