I haven't played in my big rotational save in a long time, but I recently had an idea of how to get back to it (the story was running out and it was getting glitchy). I started it with four sims about to go through university: a brother/sister pair, then potential spouses for each of them. The potential spouses hit it off with each other, so the siblings found their own love interests. I also had an alien scientist to play through the active career. Then I made some mermaids to play with, discovered mods to flesh them out, and suddenly I had a couple of occult dynasties that were all up in each other's business. Gen 2 had a storyline where the alien scientist's daughter wanted help from the astronaut hero of strangerville and a spellcaster in being more normal; when they wouldn't, she cloned their wives to keep captive, then fed the real wives bad pufferfish. Gen 3 had a Romeo/Juliet storyline between the the spellcaster/astronaut's traumatized children. Spellcaster had been going to set up his son with the mermaid baby he had raised from infancy, but the son had other ideas.
Once Gen 3 got together, I ran out of steam on their stories. The story needed something like werewolves, but at the time those weren't anywhere near the roadmap. I didn't want to stop with those sims since I was so attached, but I didn't have any ideas for where to take them. I tend to set up an end goal for my sims, then roll with whatever the game gives me on the way. My stories have been way better for not sticking strictly to the script. If I had stuck to my original plan, I would have just had two cookie-cutter families living in the suburbs instead of three occult families building towards...something.
I'm not a huge builder, but I love taking photos. I just trawl through tumblr looking for suitable houses. This storyline actually happened across 3 saves. The first I just moved into a better, rebuilt save file very early on, no story explanation. MCCC settings got away from me, so I needed to move them again. I made an evil spellcaster to play the paranormal pack, and she was instantly attracted to my main spellcaster. So that made a convenient story was to justify suddenly dragging them all into a completely different reality. I tend to get the kids to play out new packs or make a new sim and work them into the main storyline.
I never intended to let any of them fade into background characters; I couldn't bear for them to age and die off unplayed. However, I recently had an idea for how to work in werewolves and a few favorite characters into a new variation on this main story. I've been making a lot of extra background characters in CAS while waiting for Growing Together to drop, and imported a extra families that I played for a short period before moving on. Once I update to infants, I'm going to get back into this with a new save for the favorites - new makeovers, new friends, new buildings. It's going to be fun.
My other save was a legacy, so I only cared about the one main family. I kept it separate because I didn't care about the background characters, and I knew it would bother me if any of my faves did weird things in the background.