Gameplay change

Perhaps you could do a challenge. There are a variety of types of legacy challenges, or a challenge such as rags to riches or challenges that focus on one sim and the building up of their skills or having careers in the game.  Challenges can encourage you to try out and discover new aspects to the game. Or perhaps you can have your sims live for a time in one world, then when they have reached a certain stage, then move them to another world and start them in a starter home and keep playing them there, until they can build up their home, then move to another world.

Think outside of the box, and do things you wouldn't normally do. Examples; I have a sim who's a thief, he loves breaking into people's houses and stealing stuff. You could create interesting sims in CAS to spice up the neighborhood (I have many crossdressing ones). I have a vampire sim who goes around torturing people with his powers, and another who owns his own toilet store. Yep, just toilets....