The previous attempts of an online game were quite a while ago. Personally I think that if they did it right, an online addition would bring a lot more players. I definetly think it would appeal more to younger generations, which after all is their main target. Many players would love to be able to play with their friends.
I would love to see other players sims walking around that aren't controlled by my pc. Experience the true personality and quirks that each sim has. How can you possible know what they're going to do? Playing with other players in the simverse, I believe, would make sims actually smarter and more interesting.