Hmm... aside from turning it into Animal Crossing (which is actually a completely different focus from the Sims), or The Sims Online... offering downloadable content, both free (the occassional item pack)and at small prices (mini-expansions, because The Sims loves expansions)?
The problem is with how to keep true to the nature of the Sims, which are essentially player-focused god-games. Even Will Wright's latest, Spore, isn't so much online as "download other player's species to make up my world instead of having the computer make 'em up," a dynamic which wouldn't work here.
Going further, a Second-Life-esque MySims MMO might work, except that MMOs come with their own big problems too, and that it would be less Sims and more... well... cute texture sharing second-life.