While I would love to see basic weather in a future sims game from the get go, I hope that most of the time and resources are put into making dynamic and interesting sims. To me that is the base and everything else can just expand on it.
Having said that, there are a few things from the Sims 4 that I think should become part of a future base game.
1. Basic weather - Seasons could focus on seasonal activities and holidays
2. Calendar - Each pack could add events that could be put on the calendar
3. Club system - Get Together could add a lot of activities suited for clubs
4. Reputation system - Could still have a pack focus on fame
These things I feel could all be expanded upon by DLC. They already kind of do this (Get Famous added events to the calendar, almost all packs add club activities, etc), but it would make sense just to have them be part of the base game.
Anything else in my opinion can continue to be released as DLC. Pets would be nice to have from the beginning as then they could add new pet stuff in each new pack, but I doubt they'll ever do that.
