Sims 4: Please add more life stages

Well, a toddler is a preschooler, at least where I live in the USA. So that would be a pointless addition. Preschool is age 3 to 4 (sometimes 5), which is covered if you play a toddler enough to gain levels 4 and 5 in skills.  

While I understand the want for preteen, you can also do that via gameplay by keeping attention to the age in days. Child gets 13/14 days. What I do is dress them more child-like with clothing selections for week 1. During week 2 they should have skills high enough to appear older and you can dress them to look older with certain clothing selections. You can also make a teen appear younger with clothing choices and gameplay, which can help with the illusion of young teen versus an older teen. 

Adult stage is the middle age lifestage, especially if you can pay attention to the days of the lifestage, which is 42. I also do that through gameplay, the same as I do with the previous two lifestages mentioned in my comment. 

I use the in-game calendar to help me. I set up a "holiday" reminder without any traditions. Then on the morning of that reminder I do the changes in CAS for the illusion. Most times the gameplay I do is in the pace to get their skills match the illusion.

More lifestages is only going to "replace" significant gameplay. At least that is my opinion on this subject. Btw, I use the normal lifespan for my descriptions above.