I've been playing for many years and tried all sorts of stuff.
I have a few long term saves.
One is a super sim/family save where all aging is off, I have one active family, and they basically are super-powered where they try to get all the collections and content they can. The careers and aspirations they've tried are maxed. I often use this family to test out new content or try new things.
I have a couple of other saves similar to that, but with occult sims and aging on. One is with Vlad, where I was using him to learn vampire powers and content. I've also used him to test out mods and content and builds because vampires are cheaty. Sometimes when I'm feeling salty, I'll also play Vlad just to run around and wreak havoc, because he's a fun character to play when you feel like being a llama.
I've also got a werewolf save with a CAS werewolf where I did something similar with werewolf powers.
I have a long-term generational save with aging on normal. It's not quite a legacy, but it's similar style play as I stick with one family in it.
Most recently, I've been playing a new save that's heavily modded based on the Decades Challenge by CuteCoffeeGal. I say based on because I play for fun and don't follow the rules to the letter, it's more of a roleplay thing for me, but I've been having a blast with it. Using CC and mods I made over all of the townies and downloaded or made customizations of lots of Henford-on-Bagley to look like they were straight out of the late Victorian/Edwardian Era and started my sims there pretending it was 1890, changed my aging to a custom setting with a mod to make each sim year of 4 seasons/weeks equivalent to a decade, and played a family that was a YA male spellcaster, his elder spellcaster mother, and his YA human newlywed wife, dog, and horse on a farm with the simple living, off the grid, wild foxes, wild prairie grass traits and went from there. It's been the most fun I've had playing Sims in a long time! I've used clubs to make all sorts of gatherings and make my community come alive, and I look forward to updating my sims each time as time passes.