The Sims Traits

For me, seasons is an obvious answer. With seasons in base game, things like freezing to death in dorms because no one thought to put in a thermostat or open walls in a town seeing heavy snow wouldn't be a problem. Another reason I want seasons as base game is because you could do so much more with holidays, such as winterfest markets, egghunter events in the public meadow or loveday festivals in towns. With seasons for basegame, maybe you could even diversify the holiday calender, so that there was a different one for each world (let's face it, Mt. Komorebi would probably not celebrate winterfest).

Other things I believe should be base game is clubs, so that different packs didn't have to have different club systems but could build on the same one.

The basics of retail - so that different packs could expand on develop these features (so cats and dogs could contain items for a pet shop, the gardener kit could contain items for a flower shop or gardening centre, island living could contain shop shelves that fit in style with the pack...) That way the retail system could live and evolve as the game does in a much better way than now.

There are plenty of other features as well, but those three would make it easier to develop the game and expand on the ideas.