My current game follows the events of the story I've been working on for half of a year. (Clock the banner if wish to read, PM me for a link to an uncensored version). This is a no way a continuation of the story, it's just a gameplay for sake of gameplay, but sims do follow their in-story behavior and personalities. Right now I'm looking forward to starting a shop and can't decide which one - electronics of vampiric (I've never tried to have a shop). If I'll get a few good ideas, I can certainly see myself adding a few more episodes to the story.
Obviously, in this case the whole save was created for the story, not the other way around. All based on the imagination, stubbornness and determination.
First thing you need to know is that Sims can be rather dull game, so don't expect it to be exciting 100% of the time.
Second - how will you approach story writing? Do you want to play the game, take screenshots and then create a story around that or is it the story first? This is very important because you will have completely different games depending on the answer.
Third - incorporate the world around your sim into the story. Life goes on.
Fourth - try to get as clear vision of what you trying to achieve as possible. If you don't have vision, then you will inevitably run into a snag and get bored. Basically, what do you want?
Fifth - if you're making something - don't speak to anyone about the plot if you can, because you will immediately lose all interest in continuing. Create first.
And finally, the big one - outside of a very very few mods, it will all be up to you. You have to keep the story alive and it can take a lot of time. Think about it - are you willing to actually do it? It's fun to imagine it all, but to actually make it? This can be VERY tedious.
No matter what you decide, I wish you the best :)