Theme

Each expansion pack for a The Sims is centered around one or more themes.

Dating
Dating was added with The Sims: Hot Date and reintroduced with The Sims 2: Nightlife. Dating is included in The Sims 3 base game, though it is simplified from previous games. The Sims 3: Generations and the May 31st Patch added a proper dating system to The Sims 3.

Cars
Personal car ownership was introduced in The Sims 2: Nightlife, was incorporated into The Sims 3 base game and expanded in The Sims 3: Fast Lane Stuff.

Parties
Parties, while always possible to some extent, were the main theme of The Sims: House Party. The Sims 2 added theme parties such as house parties, birthday parties, wedding parties and anniversary parties. The Sims 2: University added sports parties and toga parties. The Sims 2: Apartment Life allowed apartment parties and building parties. In The Sims 3 base game, you can choose between house parties, weddings, birthday parties or funerals, and you have the option of throwing them at a community lot.

Pets
Pets are introduced in The Sims: Unleashed and reintroduced in The Sims 2: Pets. All versions of The Sims have had aquariums, fish that require a minimum of maintenance, although The Sims 3 and The Sims 3: World Adventures allow the keeping of bugs, butterflies, snakes, frogs, snails and crocodiles, as well as other fish.

Gardening
Gardening was added in The Sims: Unleashed and brought back in The Sims 2: Seasons. Planting seeds and other gardening activities are included in The Sims 3.

Vacationing
Vacationing was added with The Sims: Vacation, then again with The Sims 2: Bon Voyage, and a similar "travel & adventure" theme is the basis of The Sims 3: World Adventures.

Magic
Magic was the centerpiece of The Sims: Makin' Magic, and was reintroduced in The Sims 2: Apartment Life.

Fame
Being famous was the main theme of The Sims: Superstar. Fame was not incorporated into The Sims 2 series at all, though Sims could have reputation in The Sims 2: Apartment Life. Fame is found to a lesser degree in The Sims 3 music career, but is a major feature of The Sims 3: Late Night.

College life
College life is the central theme of The Sims 2: University, adding the young adult life stage and university campus sub-neighborhoods.

Weather
Weather is the main theme of The Sims 2: Seasons. Added four possible seasons for neighborhoods: Spring, Summer, Fall/Autumn & Winter. While it is possible to achieve some weather effects with the Fog Emitter in The Sims 3, the concepts of weather has not been fully implemented. Weather can be controlled with Karma powers in The Sims 3 for console.

Businesses
The idea of Sims managing their own businesses was central to The Sims 2: Open for Business, and was included in The Sims 3, though business ownership was simplified and businesses could not be founded independently or formed on private lots. The Sims 3: Ambitions expanded on the real estate market a bit and also made it possible to sell crafted items at a consignment store.

Apartments
Apartments could be created in The Sims 2: Apartment Life, but left out of The Sims 3. A simplified version of apartments was introduced in The Sims 3: Late Night. However, this does not include communal living, but is merely like a regular house, except that only the top floor is accessible.

Hobbies
Hobbies was a theme in The Sims 2: FreeTime and The Sims 3: Ambitions.

Luxury
There were always some high-end or luxury objects, and expansion packs would frequently add more, even if luxury wasn't one of the pack's themes. It was a theme, though perhaps not a major theme, of The Sims: Livin' Large. It was also a theme of The Sims 2: Glamour Life Stuff, The Sims 2: Mansion & Garden Stuff, and The Sims 3: High-End Loft Stuff. The fame-themed packs, The Sims: Superstar and The Sims 3: Late Night, could also be considered to have luxury as a secondary theme.