Well, the first post already stated most of mine. Though in my case, it be an entire new game.
Medieval, magic and supernatural creatures.
Class system - Royalty, Priests, Nobles and Commoners. Special interactions for all of those.
Are you a smithman, a tailor, a hunter, a thief, a clerk, a monk or an adventurer? Medieval has so many jobs for your struggling Sim.
Only the King's court wizard is allowed to do magic, but that doesn't stop some Sims who has the gift to practice in secret.
It would take your Sim longer to advance in their field of work and skills would take longer to level up (your Sim would actually have to struggle to get somewhere in their life).
Phones would be replaced with messenger pidgeons, falcons, seagulls, hawks and eagles (Vampires can use bats and Spellcasters can use dragons). And it would cost to send messages (this would cause your "friends" to stop messaging you five times a day).
Punishment! If your Sims is a criminal who gets caught (special hideout floor hatch where your wanted Sim can hide when the Knights come for inspection) or neglect to pay their taxes, they'd be thrown into prison for a number of days.
Woohooing for fun and social would be a risk, cause now you can't just decide or not decide to try for a baby, unless you visit the greenwitch in the gloomy forest for some special and costly herbs (might have to do her a favor if you can't pay...!). But avoid being seen going there, Knights will arrest people they suspect of witchcraft.
I'll stop here. I could write a minor novel about the things I want.
Sigh... the stories one could cook up in a brutal medieval life setting....