I'd be ecstatic if they added preteens, but I don't think it's likely. I think it's much more plausible that they'd consider developing the current teen life stage more. I don't think they'll change the appearances of them, however. If they were to do anything, they'd probably add new interactions and animations. The height has always bothered me, but that's far too intensive of a task as they'd have to re-do all the current animations for them. To add curfews, groundings, acne, mood swings and more would definitely be a lot of work, but I can't imagine it'd be more work than creating an entirely new life-stage with all its animations, interactions, clothing, CAS features, etc. while ignoring others that need TLC (teens and elders). Granted, I know nothing about game development, design, or coding, so someone who does is free to correct me.
I'm just of the opinion that the current teen life stage needs attention before they should implement an entirely new one. I'd love to see teen-specific moodlets, teen-specific traits (reminiscent of the toddler traits) that they can grow out of, maybe quicker decay for hygiene and hunger when teens first age-up that last for a few sim days or so that represent the growing and changing of their bodies, teen-specific whims like the "lock a door" one but for things like having their first kiss or sneaking out, curfews and groundings, acne that pops up sporadically, crushes, mood swings, school events, and so forth. This is a totally random one, but I think it'd be cute for the teens who are in relationships to occasionally worry about hiding hickeys from their parents. I thought of this one after watching my teen sim's boyfriend autonomously kiss her neck three times in a row. :lol: Maybe the parents could even have the ability to question them about their dating life which would push an embarrassed moodlet on the teen. I think having more interactions, events, whims and such for them could certainly help to make them feel more like "teens", even if they appear older. It'd be a huge plus if they could make them look younger and shorter, but I guess they decided to make them appear older due to all their more adult interactions and dating lives. I'm really hoping we'll get a Generations-like EP that'll introduce new things for all life stages, but specifically the younger and older crowds since I think the YAs have had more than their fair share of attention. I guess time will tell, but it's fun to speculate and throw ideas out there. :)