Playing origin stories from my main save has kept me playing one central family in a couple of saves. I'll harvest sims from my main save, with too many sims and stories going on. I'm planning to play out everyone eventually but the time it takes rotating...I've realised I can't do all stories justice so I transfer them to other saves or their own. I've done this a few of times.
Recently in my large save I've been enjoying playing adult Zoe Patel and her family so I transferred them to their own save. Now I'll play out the "House of Patel" fashion empire inspired by the new Gucci movie and the Fashion Street kit. I'll be starting Zoe in toddlerhood, giving her siblings, and going into her parents background before they started as fashion retailers. Zoe will be competing with a brother to run the fashion empire when she gets of age and after she goes through uni(never did uni with Zoe in the other save). Not sure if I'll have her marry Salim again(I might because they already have tons of photos in the main save :p ).
Using sims from other saves and starting them at a different life stage and either following their pervious story or giving them a new path might keep you more intrested, especially of you've played a sim to old age or death. I've been able to stick with a single family longer than my norm doing it this way.