I always love giving my Sims, or founders a background story of why they're a founder, or why they're doing a specific challenge. I don't necessarily write out a full story for my saves anymore (I did back in Sims 2, because the story album was so convenient and actually a lot of fun to create your own stories and then share them on the Exchange.) An example of this is with my last 100 baby challenge.. I gave a background story of her meeting the love of her life... or so she thought and they were going to explore the different worlds together and instead of going off to college right away, she was going to travel with him. But instead... he left her a note, took all her Simoleans, and fled to his next adventure. So, she vowed to never let someone get close to her again, instead they would fall in love with her, she had the Klepto trait, so she would give birth, swipe something from their place, and then "call it off" and tell them that she just wanted to be friends. Then she'd move onto the next person, until she found the one that took everything from her. :x
This would allow me to still play out things in the game that are available to the Sims involved. But if it were like a legacy Challenge, I'd still want a small background story, but I'd also want in any save that I play to kind of give little scenarios to Sims that I'm really enjoying playing so that I could play them out in game. Another example is with that same 100 Baby Challenge, she ended up having twins (without any fertility treatments, or lot traits), and one was a girl and the other a boy. Once they were teens, I noticed that the male really enjoyed music and would play on the instruments a lot, and so I made some "extra" Sims for him to have some friends to make a band. While we do not actually have a band system in the game, we did have instruments that they could just play and practice around with. So.. super cliche, but also, I had the sister learn the singing skill so she could join the band, too. Lol
I just like to plan things in my saves. Even if it is just making extra Sims for my main Sims to get to know and form "best friends" relationships with, or to have Sims get into romantic relationships and start their own families. I like to plan out get together's, BBQ's, holidays, and little things like that and that's why I love the calendar feature so much because I can now do that in game instead of on google documents, or some other app thing. :)
A great topic, by the way.