When it comes to apartments they have said that they could add them in future packs as there is nothing stopping them but they would limit just how many apartments they add.
As for previous gameplay items from other packs like hottubs the have said that if their were to add more hottubs they would have to be different like inground hottubs. As for laundry no it wouldn't be added in any other packs as laundry day covered two ways of doing laundry. Manual and the standard machines. If laundry day features only manual then they would be able to add machines later. Actually they could keep manual just to laundry day and add more machines/make them base content but I can see many players going into an uproar over laundry being made base or being added in other packs as laundry got a lot of lash in ts3 and many players were vocal about not even wanting the option of laundry in their game in this iteration.
They have shown though that they could make features base like photography and most recently social media but for the former a lot of players complained that a feature they paid for was now being given away for free but photography and social media are features that tie into new content and help expand the possibilities of what can be added in new packs/patches by playing off those features.
Overall though the only way gameplay features from previous packs can be added in new packs is if it can be changed enough to make it unique in itself. To be made base it needs to play a big enough role that without it future content won't be able to be added properly. Most notable is how the style influencer career that requires photography and plays into social media. This is most likely why that career was patched in and not a part of GF.
