Secondlife can't be optimized well because it's user made ... people can bring in their creations... that means textures are done badly ect. It's a crash fest waiting to happen.
I would probably give an online version of the Sims a chance if it was made. If it's done I think it should be done in a way that people can have private servers with limited or friends/invite only possibilities. The owner of the server could determine the mods to be run.
There could be larger public servers also. There would be a huge possibility for trolling.. especially if people wanted to play with things like vamp powers, fire ect. Could be fun but we all know how that goes. You have servers set up basically to bully people that come in.. all well and good if your prepared I guess or if you agree to it. Would be nice too if there were public servers that had some policing done.
I don't see an online Sims game being rated for a general audience. It would not be a good place for kids.