I'd LOVE a preteen lifestage. There would be sooooo many cool milestones to go along with that age bracket. First crush, for example. Outside of perhaps ace individuals, who doesn't remember their first serious, paralyzingly scary crush? Things like getting their first hormone induced break out would be a cool, realistic random event. Perhaps an asking to be allowed to date type interaction. A random event where your preteen wants a style change because they are growing up and want to look more mature. Basically, I just see a lot of room for new, interesting gameplay with that lifestage.
I have my fingers crossed for a Generations style GP or EP. One that fixes some of the odd quirks to the current lifestages, such as children that stay up and out alone to all hours of the night hitting up nightclubs and a lack of curfew for teens and children alike. New age-appropriate venues would be SUCH a blessing. Arcades, skate parks, self sustaining malls/shopping centers for teens and preteens, parks that actively attract/spawn other children for your sim kids to play with, a kid-friendly restaurant option so that we can build a Chuck E Cheese style eatery that kids will actually get up and play at in between bites of pizza. Places like that.
But yeah, I would love family play to be expanded upon and I feel that a preteen lifestage could easily go along with that and really make it something special.