So the first place my mind went was not only being able to knit, but being able to knit things for baby Sims and have them be usable, so they could finally have a different look. Sweaters, hats, booties, etc. Thinking of a pregnant sim knitting for her little one makes me go awww ? Knitting blankets would be fun, too, or scarves. I think the thing I'd really want to see with this hobby is whatever your sim creates is actually usable by them too.
Also, in addition to hobbies, I would to see the return of more clubs/after school activities. Swim class, ballet, karate, something akin to 4H or FFA, chess club, cultural appreciation club. Honestly, there are so many options!
I would be happy to see pretty much all of these, especially if they were in a pack like Free Time!