I came up with this idea while browsing the forums, and I thought it would be a perfect idea to post it! Feel free to add on and share your opinions.
CAS would include lots of clothes for children and even more for toddlers, more clothing good for teenagers, a few new kinds of facial hair that could be seen on a teenage boy, and some clothes good for elders. And the ability to create two new lifestages- babies and preteens/tweens!
Build/Buy Mode could include new activities for kids, and not just the drawing and science tables. Actual, completely new things! Lots of child, teen, toddler, and elder specific items could be added.
The new would could be inspired by places in England, I think this is fitting because the only livable worlds that aren't stereotypically American (coming from an American) are Selvadorada and Windenburg, and I don't see a family pack featuring a world from anywhere but North America or Europe.
Babies can now officially be made in CAS, though they're not quite out of that crib just yet. Babies are still unplayable, just like Cats and Dogs, but the cribs, despite taking up only one space of the floor, now have a space of 6x6. There are changing table objects, baby bath objects, and highchair objects that can all be placed inside that 6x6 space, and they are all usable. This does not free the baby from it's crib, but also makes them much more worthwhile. The lifestage now lasts 5 days instead of just 3.
Preteens/Tweens are right inbetween the teenager and child lifestates. It's length is inbetween toddler and child, lasting around 10 days. They go to middle school and have new, special interactions such as "Attempt to Impress" for teenagers, "Tease" to babies, toddlers, and children, etc.
Elders should receive a small overhaul, which includes a new venue type, retirement homes, that they can go to as soon as they retire from their jobs until death. There is also a new game, bingo, which can be played on the new bingo table.
The school system is now active. It's pretty simple, but adds a lot more depth to the game. Children go to Elementary School, Preteens/Teens go to Middle School/Jr. High, and Teens go to High School.
New events, such as prom, graduation, and sleepover could be added. I also think it would be super cute if couches got a new interaction, called "sleep on", that just gives less energy than a bed would but more than a nap would, along with a couple blankets on top of whoever is sleeping.
This is a link to a thread that I think has great ideas as well, though it might make this to full for an Expansion Pack: https://forums.thesims.com/en_US/discussion/964947/divorce-having-a-bigger-impact-on-families-why-how-and-what-it-should-involve#latest