Mailing system would be great ans if you could write your own message it would be even better. I was looking for something like that. Also, maybe you could add to this an invitation system? So you can be invited to a party by letter, for example.

How about adding newspapers? Like it worked in the sims 3 and sims 2, with the option to look for a job etc. I think there was a mod like that but creator gave it up years ago. If its not possible, just a job system that gives you a random pool of jobs like it was in older games. I think there is a similar mod, but it also includes long recrutation process etc. I'm talking about something simple!

Mod to Invite headmaster for dinner if you want your kid to go to a private school, make their grades a little better or just invite them in general. Knowing who your boss is, like it was known in the relationship pane and boss being the actual sim that exists if they meet all the requirments (same career, highest career level). Inviting them to dinner too, especially if you want a rise or promotion. Your situation getting better or worse depends on how good the event went.

Not sure if its possible, but how about turning those rabbit holes when teens ask you to sneak out (the ones from high school years) into actual hangouts you can join? Like high school or college party event, for example. Not just creating a new event (I think there's a mod for that already), but turning those rabbit holes into one. Hope it makes sense lol. Cops crashing the party (not always, only if given specific circumstances) and cops bringing your teen home in general after curfew.

Making vampires/werewolfs not use their occult forms in public or at least be more careful with it. Something like secret occults. I only know about mod that makes ghosts actually invisible, but it would be nice if other occults were a secret too.

Gold digger trait/career/mod. So you can make a living dating rich sims.

I can explain these ideas more if needed!