I'd like a cross between TS1 Superstar and TS3 Late Night. So I'd like:
Fashion:
Model career. A catwalk. Fashion design career (like the stylist from Ambitions). Sketching. Also the sewing machine from TS2 Freetime. New 'couture' or 'high fashion' clothing category. Celebrities and snob sims would be more likely to talk to sims in 'high fashion', and you'd have a better chance of getting into venues.
Film
Acting and Writer/Director branches. Sims could write plays, films and t.v shows and have them acted out by actors on stage or sets, like the bard in Medieval or the actors in TS1 Superstar. Instead of having premade sets, maybe we could decorate the sets ourselves like the show floor in Showtime.
Musician
I imagine this working in a similar way to the Late Night bands, only they should also have a singer!!! You could mix and match instruments to make your own band, so if you want a singer with three guitarists, or a string quartet, a jazz band, or a barber shop quartet, you should be able to do this. So I guess they would need set songs where each instrument is recorded separately, so they could be layered depending on which instruments you select for your band. Like in Late Night bands should be able to perform at venues with a show floor, or even randomly around town. Then I'd love the recording studio back from TS1 Superstar. Also the sets and video cameras from the film career could be used to create music videos too! \o/
Celebrity System
Similar to Late Night in that you could be a celebrity for any reason as long as you have enough fame points and connections. So you could be a celebrity chef, or even a popular gamer or 'simtuber' as suggested above. Only, I agree with the comment that there needs to be some kind of opt-in/opt-out feature for being a celebrity per sim. As I don't want ALL my sims to be celebrities. Celebs should get perks like free stuff and being able to get into any venues.
Exclusive Venues
These should be customisable by the player and work in a similar way to the clubs. Any venue should be customisable. Maybe we do this by adding a rope, bouncer, ticket machine, or special door to the lot. So if we want to create a gym exclusively for gorgeous celebrities, we could do so. Or if we want a restaurant only for rich food-snobs, we could do that too (oh... we also need restaurants).
So yeah, if we got a celeb expansion like that I'd be thrilled!!! B)