Start off by knowing yourbasic plots and then dramatic situations :)

I'd say put dragons in it. Everything is always better with dragons.

Now, more seriously, I think knowing the storyline prior to coding is ideal. I'd say, write down what's happening and why. Develop your universe before you start coding, since the setting can really make or break an RPG.

And, above all things: Dragons.

Try to avoid mindless cliche. Don't just put elves and orcs or whatever in your setting just because that's how other RPGs do it, and on a similar note, minor twists on cliches don't really accomplish very much (e.g., Dragon Age's "Our game is daring and different because the elves are ghetto trash!"). If you must indulge, come up with a strong justification for doing so.

You can't surpass the story of "Planetscape: Tournament" in terms of quality and verbosity, so you might as well give up now. :p

Seriously though, most RPGs follow the same tropes so your best bet is doing something completely fresh and different. I'd buy a game where your main quest is to take care of your small community instead of that nonsense about saving the world. That's what got me disappointed with the story of Fallout 1 and 2 – in the first half it was all about your community, but they just had to put in saving the world because everybody else is doing it. I'm tired of saving the world. To be frank, a game that deals with much more ordinary and mundane tasks all the way to the end would really be a refreshment.