Which aspect must be BG part in TS5?

Seasons/weather should've never been cut from the base game with 2 in the first place. They won't change though as it's been proven that people will not only buy it despite it being cut from the base game, but will even praise it... (See below for reference on it being planned for the base game)

Weather was originally planned to be an element in The Sims 2, The Sims 2 preview at E3 2003 demonstrated rain and lightning effects. However, it suffered from technical glitches. Rain would fall inside sim's homes and Maxis was unable to resolve it before launch.