I've only played TS4 since the spring of 2021, so what passes for a "long-running save" for me is nothing like the epics some of you have created. My daughter and I have what we call our "story save," which features Cameron Ross, the Sim who appears as my avatar. His parents, siblings, and grandparents are all part of the story. That save is on hiatus, while we make doubly (triply? quadruply?) sure that the various patches, etc. that have been released don't wreck it irreparably. One of these days, we're going to have to load it up and take our chances.
The "story save" is set in @SweetieWright_84 's save file world, which Z and I dearly love. We play the "unpopulated" version, because Z enjoys creating Sims in CAS as much--or more--as she does playing the game with me. The "story save" is filled with a variety of Sims; we've created many of them, and we've downloaded others from the Gallery. Some of @SweetieWright_84 's Sims have roles in our story. Only two or three premades exist in this world.
Z and I also have another two or three saves available. We started them as "test saves," in which we try out the features of a new pack, or experiment with the game in other ways. They've become so much fun that we keep playing them. We cast a variety of Sims as lead actors in the "test saves." Sometimes they're Sims who have been supporting actors in other saves, finally getting their chance to be stars. Other times, the featured Sims are new creations.
We always put the Ross family into each of the test saves, too. We're attached to them, and we like seeing them around or having our lead actors interact with them. However, we won't let the new leads become romantically involved with Cameron or his girlfriend, a cute Z creation named Angie Reynolds. The first time one of our Sims flirted with Cameron, Z was not amused. "OH no," she declared. "That's not happening." Now we put Cam and Angie into the game as an engaged couple, and so far, we haven't seen them stray.
This thread has inspired us to start a new "test save." Z has some freshly created Sims she'd like to place into a story. :blush: