You know, you could use Legends mode to build a world to frame your plot inside
I actually did that with my last big fort Woundtorment. The best way to start is to look at the statues and engravings from the area that describe events from the time before you embarked. Find your "local hero(es)" and read up on them, their families, the beast the killed and how they died. with 250-500 year of history there will be some interesting stories for sure.
My local heroine was a human hunter named Guleg Laborbunnies. Her husband was killed by a werelizard. She went on a journey and began hunting giant wild beast until she found the werelizard, Hustra, a female human. They fought but Hustra killed Guleg. Ancient history of my embark site.
After all this years Hustra was still alive and terrorizing the area though. She had started her own "family" if you can call it that, spreading her curse among humans in the area. In the first autumn the merchants killed Das, one of her brood. During the winter one of my miners was surprised by another werelizard but killed him with her pick. Bleeding and with a severed leg nerve she dragged herself into my hospital and we patched her back up - after some precautions obviously.
She recovered well, became a legendary crutchwalker, hunter and marksdwarf. She earned herself a name as slayer of beasts and cursed monsters and became one of the heroes of Woundtorment.
2 more werelizards appeared but my war dogs drove them into the river where they both drowned. We dug the bodies out of the ice during winter. I knew Hustra was still in the area and alive. We were waiting for her, building traps, training dogs and militia.
Finally, after many years Hustra attacked herself. We baited her to the fort and delayed her until she turned back into a human. The trap was sprung and we caught her. She spend years caged in my great hall before my marksdwarfs killed her as punishment for her crimes. I imagine my "Slayer" enjoyed her revenge for her now useless leg.
The stories are there. You just have to find them and integrate them into your fort. Werecreatures, vampires and necromancers can have long and often interesting histories... =3