Not so much babies (infants) that need more interactions, but toddlers!
I don't care so much for clutter in my Sims 3 game. I have that in real life and would rather have a nice and tidy home.
I absolutely agree. I miss having toddlers, splash through puddles, steal other toddlers bottles, ask to be fed, read to etc. Also, I wish they could have more traits too. Toddlers should be able to bite, hit, pinch their peers, as well as steal their toys. Hehe
EDIT: I also think it would be sweet and more realistic for toddlers to also climb in and out of their cribs, eat the solid foods that the child sims or older eat, in tinier cut up portions, climb on top of the counters, break furniture, have some sort of contribution to the (major) messes made, by doing this, even mess with the older siblings' and parent's important papers? (homework, bills etc)
Babies on the other hand, need some sort of free will. They should also be bathed, and grow out of their blankets after a day. They should crawl around, and their parents and teen siblings should be able to tend to them while sitting down watching tv.
EDIT:
I like the idea of pets having their own traits, but really they should have their own pet-related traits. Some of them would definitely overlap, but I would find having a kitty cat with "Natural Cook" and "Handy" somewhat silly and extremely unrealistic.
Training is another issue that I see. You would have to be able to train them faster on the shorter life span settings, or else sims will never get them trained in their lifetimes (or they'll be trained unreasonably fast on longer life spans). Another thing I see me having a problem with is pet mortality. Nobody lives forever...unless you turn aging off.
My idea would be the traits available to humans that are typically available to children sims or younger, ones that wouldn't be beneficial for humans to learn skills with. Pets would have the Good, Evil, Friendly, Loves the outdoors, Hates the Outdoors, Hydrophobic, Neurotic, Insane, Loner etc.
Dogs or cats with the good trait certainly won't be able to donate money to charity, but they certainly can brighten their human master's days, by giving off their cute face. Evil Pets, like evil toddlers, can sense when their owners or other pets are suffering, and get delighted by it. Mean-Spirited pets exist as well. Like Mean-Spirited humans, who like to fight, argue, and be rude for no reason. Mean Spirited pets can be vicious to their fellow pets, and hiss, growl and attack their owners for no reason. My childhood cat was a loner. She liked crawling in dark places, and didn't care for the company of humans and pets. So a cat with the loner trait is very realistic. My giant Persian orange tabby I have now, could certainly fit the Absent-Minded trait, as well as the Neurotic Trait, he's slightly a little messed up in the head, and not always there, and will freak out and run around in circles every now and then. Pets can certainly be grumpy and annoyed as well.