What kind of event would bring every PC and remotely friendly NPC in a given setting together?
What would it look like if every major not-immediately-hostile character in the setting met in one room?
The Council of Elrond in The Lord Of The Rings and assorted councils of war in various epics, the ball at Netherfield in Pride And Prejudice and The Prom in Buffy, Friday night at Rick’s in Casablanca, The Five Doctors, Poirot calling all the suspects together to reveal who the killer is, one of those mass superhero crossover meetings that you really need someone like George Perez to draw...
One-off salon-style LARPs such as convention freeforms like Blood & Betrayal tend to focus on such events, but this has me thinking about such big social and/or political events in an ongoing tabletop game. Social events and political gatherings will have different feels, but both have plenty of room for multiple agendas, feuds and conflicts, and ways for the PCs to get in trouble.
Vampire: The Masquerade starts with one of these. Baptism by Fire in the first and second edition rulebook puts the PCs and all the vampire NPCs in the sample setting in a room together for a party at the prince's request/demand. This is admittedly only seven NPCs, but it sets a significant precedent, particularly for the live-action version.
The hard part in a tabletop version is keeping all the NPCs straight. If there’s a player you can draft in to play some of them, that would be a big help, particularly when you have two or more NPCs in a single conversation...
One important question for such an event is who the PCs would want to avoid. And whether the players would want them to be able to. If Ilsa had managed to avoid Rick, Casablanca would still have some great jokes but not much of a plot.