Not sure how unpopular, but I see so many people wishing for it and I couldn't care any less - Tweens.
I absolutely don't need a life stage between children and teenagers. I don't even understand what the difference between teenagers/children and tweens would be. And to be honest I don't think I even heard the word "tween" before TS4 or I'm just too old.
And if they ever actually consider adding it then I really hope they've actually fleshed out all the other ones before. I would rather have more depth and differences for the existing life stages than adding even more useless ones that are basically just different because they have a different height :|