I analysed some packs and created a list of features that would make The Sims 4 base game look more like the previous The Sims base games. Most of them aren't key features for their packs, others are...
Gardener career (Seasons): Well we have gardening skill/gameplay and also a community gardener NPC. So having at least the base career (without the 2 branches) would be great to make us be able to work semiactively as a community gardener NPC;
Simple Living (Cottage Living): There's not much Cottage Living can give us to make it a pack exclusive feature. It also fits many other dlcs like Seasons, with the seasonal gardening, Island Living, with the intent of making a Castaway gameplay, Horse Ranch, with its ranch lifestyle, and the base game that have lots of plants that we can grow and eat ourselves
Free weights/Dumbbells (Adventure Awaits): I still feel very sad having a whole gym stuff pack and it only brings one new exercise machine and that's big and chunky climbing object, that isn't unique anymore since we can do climbing with Snowy Escape and the personal/home gym part wasn't touched. Also the fact that dumbbells were showed in Lovetsruck ads but didn't come with it really annoys me. And finally thefact that we still need more sports and dumbbells are really basic to exercising.
Park worker career (Adventure Awaits): It totally should be a feature to add depth as for Outdoor Retreat Game Pack that was the first pack to bring camp related activities and forets rangers as NPCs. And for base game it'd make the national parks more useful and different from the normal park.