I would say the Wuxia-campaign I currently play in.
It started as a one-day oneshot when the GM and one of the players, were out at that player's family summerhouse back in 2020, as they tended to be every summer, due to them being childhhood old friends. It started as small domestic drama in the small town, where the player character (a young noblewoman from a minor house), found ruins of an old abandoned temple outsie the town, as well as fell in love with and "stole" the most attractive young gentlemen form another minor noble house. Some rivalry with a third house.
The system used back then was Tianxia: Blood, Silk & Jade. We did however wander away from that even in the beginning.
Then, when the GM and the player came back from that summer house, the GM had created his own setting; a huge empire consisting of 5 different countries, where 4 of them took turns sitting on the Moon Throne which was held in the 5th country, located in the center of the empire, which was the administrative center and had the connection to the Celestial realm. This was how things had been since the empire had started 5000 years ago. Now however, some strange things were staring to happen. The GM made the supporting characters for that the group with the original nobelwoman. My character was then the fiance, a pretty nobleman who was a scholar. Another there was a lady-in-waiting/friend of the noblewoman, and one other that I can't remember. Other characters joined later, as not all players were availiblae in the beginining due to summer vacations etc, (the group consists of 1 gm plus 7 players, but one has left the group and been replaced by another since the campaign started)
We also made our own characters, that ended up coming from the different other countries. And the GM' creating the supporting cast for each. So we had 4 distinct groups, one in each of the outer realms. Then the GM also created a group of sibling characters from the 5th realm. They were out for revenge as their noble house had been massacred when they were children. Each of the characters we made ourselves were the main character in that group. And not all of the characters are noble upstanding citizens. some are criminals of various kind (one assassin, that actually killed of the original character and her lay in waiting. they were revived though through a miracle, and direct intervention by gods. The assassin, got some divine intervention and repented, and being accepted as neccessary as his fate was linked to that of the others.
Each of these groups had their own storylines, and we would find clues here and there that things were wrong in the empire. We play one group for a while, then switch to the next group of characters. We have also had a bunch of other smaller groups of characters made by the GM where whe have done small one-shots, when not all players were present, and the GM either wanted to test stuff, or there were side-missions that the main set of character could not do. Some of these would be by characters opposing the enemies hundreds of years ago as well.. I think we were up to 11 or 12 different groups of characters now.
And all the stuff the various groups have done has tied together into one huge plot. Some of the groups of characters have met each others. We are facing a huge threat that the enemies has been planning and working on for hundreds of years, carefully corrupting society, and secretly destroying things. Even the gods have been affected. We are getting into the end stretch with maybe half a year left of the campaign or so.
During the campaign we switched into a PbtA-based system, and now back to using using Fate Core.