Get to Work: Either more active careers or making more of the existing careers active. Or an approach to the retail feature that made it more rewarding to just have your Sims be the shopper rather than the business owner. Also, some sort of expedited aspect of shopping so you don't have to jump through hoops to make a profit in your business. And also a robust simulation of owning a business without having to always be playing the business.
Get Together: More dancing styles (modern, ballet, tap, swing, etc.). And a dancing career. More communal activities for teens and younger; like extensive amusement arcades or carnivals or some sort of mock-Chuck E. Cheese place where kids can go to have fun. This was the perfect pack to add a lot more gameplay for non-YA/Adults, and it fell short in that regard.
City Living: Either a more extensive apartment editing system or more festivals as others have said. A skatepark would have gone great in this pack also. Also rollerblading or using a scooter or bike to get around could have been introduced here.
Cats and Dogs: birds, reptiles, in general small pets both conventional and unconventional.
Seasons: Figure skating career.
Get Famous: More athletic careers or skills/activities (because athletes are celebrities too) like tennis, gymnastics, golf, etc. A more elaborate filmmaking career to include Directing career, Cinematography, Editing, Sound Design, etc..
Island Living: N/A as I don't have it.
Discover University: N/A as I don't have it.
Non-pack specific but I'm always wishing for an in-game way to disable the cellphone. And also I would like to contain certain Sims to certain worlds. And also neighborhood traits that are sort of like lot traits but blanket over multiple lots, so you can still have individual lot traits while also sharing other traits with other lots.