I think that coming up with a plot for a story in the Sims is the hardest things to do. I think it's better to have a story-arc than a story per-se as the Sims themselves are pretty awesome with telling their story themselves.
I mean, plots like this:
-Sim cityslicker who wants to become the jockey and takes up living in Appaloosa plains.
-Investigator in Bridgeport, uncovering a vampire-conspiracy.
-Pick a challenge, any challenge, and weave your story around that.
Pretty soon, you'll see little stories unfold around and with your Sims and it'll snowball from there. At least, that's how it works for me, I let the Sims do the work. ^^