Okay so I know this is an old forum, but when I look up "Sims 4 Storylines," this is the first thing that pops up, so I figured I'd share mine.
So basically my original main character is named Kaylee Stewart and she lives with her dad and little brother. The idea is that her mom left when she was 4 years old, and they haven't heard from her since. Anyways, she is a part of the Paragons, and is very close friends with all of them (including a self-added member; her best friend.) So she is living a pretty normal life as a high school student, when there is a new student at the school name Lucian French. Basically, the usual happens, they meet, become friends, and fall in love. Then Lucian's rival, Vladislaus Straud, finds out about Kaylee and tries to attack her, but Lucian shows up and causes him to leave. That was when Kaylee learned the truth; Lucian was a vampire. Basically it took awhile, they fought about it, but in the end, she forgave him for lying to her and accepted him for who, or rather what he was. But she made a decision that she did not want to be one. They continue to date for a few months (not actual sim days obviously, I'm just kind of pretending it was a few months.) While they were together, they were unaware of Vladislaus' plan. So basically Kaylee and Lucian were getting back from a date and he walks her home and leaves. While they were out on their date, Vladislaus had gone to her house and drained Kaylee's family of blood, then lit them on fire to cover the tracks that they were murdered. She came home to find them dead, while Vladislaus was watching in the dark corner. Then he flew out as mist. So let's fast foward like 2 months or so (again not actual sims days). Kaylee had taken time to mourn and accept that her family was gone. She still didn't know that it was Vladislaus though. Basically following Kaylee's family's death, Kaylee and Lucian got closer. Eventually (even though they were teenagers) he asked her to marry him and she said yes. When they got married, they moved in together in a new house. They both graduated high school (I made Lucian age up just so I wouldn't have to have him skipping school everyday as a teen). Then Kaylee found out that Vladislaus was the one who killed her family (that part is more detailed but I won't get into it cause it right now). She went behind Lucian's back and created a vampire cure to cure Vladislaus with. She stupidly went to Vladislaus' house and confronted him. They fought and she lost. Vladislaus decided as punishment, he would turn her into a vampire. So he did and then when she was still in transition, he compelled her to forget it. Meanwhile, Lucian found the ingrediants Kaylee had used to make the cure, at the bar and immediately went to Vladislaus' mansion. He found Kaylee lying on the floor unconscious and grabbed the cure out of her bag and cured Vladislaus. Then he took her home. They were both unaware that she had been in transition for awhile. Then Kaylee got pregnant while she was in transition which would cause some complications (I'll get into that later). Eventually they found out that she was in transition and she became a vampire. They assumed that the baby would be a vampire as well since she was a vampire now and there wouldn't be a 50/50 chance (or so they thought). Then she had the baby. That's when she realized that because she got pregnant while being in transition, that the child would have a 50% chance of being a vampire and being a half vampire (ik that isn't actually a think but I'm just going to pretend).
Anyways, that's as far as I've gotten so far. I hope you enjoyed the story and got some inspo for your game if you want.