I’ve worked from the other direction for the most part (or perhaps gone from both directions at once). Like a Lisp program.
I use modules heavily so when I get a new module, I’m like “OK, so these guys are sorta like the al-Badia so I’ll just have them be al-Badia, while these are like The Old Ones so I’ll have them be the Old Ones, and these other guys are blood cultists and maybe they can be part of the same blood cult as the ones from that other module” etc. A suggestion I first picked up from Prescott’s Trilemma. It makes everything super interconnected.