I think if all the owned businesses (restaurant, vet, retail) could all be improved and made to function similarly that'd be a big help. It's so weird how you can own and work at a vet, but when you own a restaurant you can't work there. Even parts of the UI functionality are different between businesses. So yeah a big thing for Dine Out obviously would be letting your sim(s) do some of actual jobs (my goodness I want to help show people to their tables sometimes when there's a backlog) but I think all three (so far) businesses need to be improved and stabalised, while we're on this topic, so maybe that's less of a pack refresh and more of a general game improvement update.
Refresh-wise I'm tryna think more along the lines of what they did with spa day. Improvements not just for one side of the transaction but both - restaurant staff but also customers. Being able to order appetiser meal, main meal and desert (and drink) all at once and have the next delivered after the first is done. Being able to try to dine-and-dash could be fun: TS2 had the ability to try and skip your bill by hiding from the resturant staff for a set amount of time. Being able to have that embarassing birthday moment where the staff bring you a cake and sing. And on the resturant staff side, maybe a kind of home delivery / drive through / online aspect where you can make meals to be sent to off-site customers - this would help increase income without having those unmanagable sized crowds in the resturant itself.