I honestly feel like preteens would be great addition. First of all the teens that we have now feel more like 17 to 20. They can have romances like adults with the exception of their woohoo being called mess around and having a few more romantic options. Kids should have their little crushed, but I feel as if a shy kiss could be implemented better with a preteens life stage. Preteens would look more developed than a child, but less developed than a teen. Making the growth spurt from child to preteen less drastic. So basically by adding preteens it would give the player more adult options without fully leaving the kid options. This is exactly the way I experienced growing up too, I wasn't developed fully, life was awkward, I wanted to grow up but not exactly. I feel like preteens would give the sims team the exact opportunity to make this part of life happen for the sims 4. :)
Edit: Also there's other things that teens could have to make them differ from the rest like graduation, prom night, awesome parties while parents are out of town, learning to drive if we get cars back, etc.