I usually give back stories to the sims that end up in my game. I don't play with most of the original pre-mades but I have some "new" generations of several. Of the ones I've done anything with, I have the following:
Goths - Old Romanian/Germanic - I have thirds cousins who were contacted by an attorney to inherit the Goth mansion "Downton Abbey"-style. "Hello, you are the only living relatives so you've inherited this mansion." I have characters named Stefan and Gunilla Goth who moved from Europe to live in the Goth mansion in Willow Creek. They were introduced to each other and moved in as teens so they had a fancy house but very little cash. They live there with fluffy cats and now twin toddlers as the only surviving Goths. Stefan was raised in Switzerland and is a Swiss German with distant Romanian ancestry. Gunilla is Romanian and is an expert classical musician. They are intense and very gothic. I enjoy that household.
Landgraabs - Dutch - I brought back Malcolm Landgraab as though his parents died in a plane crash. He has a younger teen sister Sophie. Malcolm is materialistic but not a bad guy. Sophie is very popular but also nice. Sophie is currently dating Paolo Rocca's son because Paolo moved to Oasis Springs and lives in one of the suburban houses.
Von Haunts - Euro royalty - I have a castle where the most recent Von Haunt heir, Alistair lives with his wife, Claudine. He is a prince and she is a princess. They live in a huge castle on one of the largest lots in the game. Their son, Prince Lionel, is the heir to the castle and highest title in the game. Their house is so huge, they can completely avoid seeing each other all day. Alistair has had several affairs and is now in love with Sergio's daughter who is an aristocrat because her mother is Luna Villareal. I think Alistair and Claudine are getting divorced which is a scandal. Claudine looks a bit like Princess Diana. Whatever happens, she'll be very glamorous.
Villareal - Spanish aristocracy with some French ancestry. I also thought they were from that region. Hugo is Sir Hugo in my game. He's nearing death and his rebellious son, Juan Hugo, is his heir. There is also a sister, Leticia. Sir Hugo has a beloved large hound named Matador who follows him around.
Delgatos - I've decided that the dad is Brazilian and the mother is Trinidadian. I decided to move them to one of the larger houses in Oasis Springs where I have a nice community pool. I concluded that they did not like the weather in Brindleton Bay, preferring a hotter climate. I know Delgato is Spanish but is a lot like Delgado. I might change the last name to Delgado. Anyway, they are Brazilian & Trinidadian in my game.
Rasoyas - Indian - In my game, the Rasoya family goes back to Indian royalty. Raj is long-dead but has a bunch of children who are from the Indian diaspora. One of the grown up children is very socially active in politics.
Bjergsens - Scandinavian - Bjorn and Clara divorced early. Bjorn really played around but married a Frenchwoman in my game. Clara was highly independent and did not remarry. Bjorn was one of the most fun characters in my game. He was kind of "James Bond". Anyway, he died a long while ago. Sofia is a hippie in Oasis Springs who is nearing elderhood and has two artistic sons - one in art and one in acting. Elsa is a doctor and still lives in the original house. Elsa has kind of a perfect life. I haven't played her that much lately. I usually play households with more going on or more complex personalities.
Siobhan Fyres - She's an independent woman in my game and still lives in that massive house. She has two adult children who have currently both moved out. Siobhan has a retriever that she loves. She actually enjoys living on her own with her dog. She was the first sim I sent to Mt. Komorebi on a vacation where she became good at skiing. She's the kind of woman who wants a glass of wine and a book by the fire with her dog. She has been hooking up with Sofia Bjergsen's son who is a YA artist who finished university. Siobhan wants no commitment there but likes him. She is in "me time" after having successfully raised several children and after divorcing a long time ago. Sergio was just someone she dated in high school. I think she will travel more before she becomes an elder. I like playing her as a solo household.
There are other pre-mades in my game but they intermingle with other sims and it's really up to me how much I play them. The sims I add to the game all have origins as well. For example, Glimmerbrook is an Irish village in my game and I've moved in several Irish families. I'm a realism player so the magic is a rumor based on Irish legends & stories. The pub is an Irish local pub in a small village. The frequent heavy rain is ... Ireland. I think the setting could also be Scotland but I wanted to add some Irish and I thought this is the closest that we'll get. I know it can be North America but in my game, it's not.
I brought back the Grims as descendants of the owners of the quarry. I haven't played them yet, but yes, I sometimes add characters based on back stories about the worlds rather than directly playing with the pre-mades.