I got a bit of an issue... one of my players lost his character in the middle of a quest (they are traveling to a dungeon where a supposed artifact is being held. The character died in the middle of the trip when they were ambushed). He wants to use a new characters, but I am out of ideas on how to make this new character join up with the party when they are currently on a secret mission...
Take a pause from the current mission at an easily remembered point, like the end of a good tv episode. Next "episode" is a flashback that introduces the new character, who the characters met years or months ago. If the party hasn't known eachother that long, then have most of the group use one off characters that you can later use as NPCs if you want, or they can pick up if a character dies.
Do a shortish, one off adventure where the new character proves their worth, and their loyalty, to the PCs that are in the framing story (the part that isn't a flashback).
Then, the adventure ends, everyone parts ways for one reason or another, and the adventure after that, goes back to the mission, and introduces that old friend at a critical moment, which you've written to be very early in the "episode" perhaps the first scene.
Perhaps they are an agent of the person who gave them the mission, perhaps they are working for the enemy, but have just realized that they shouldn't be, perhaps they are an escaped captive, perhaps something completely else. Either way, don't use an IN that makes that player wait through half the session before being introduced.
I really need to finish my rules for letting a PC's death have Heroic Sacrifice benefits, and letting them "force ghost" on behalf of the remaining PCs, creating a feeling like in movies and shows where Wash dies, but it happens while he achieves truly incredible piloting, and gets the group through something they had essentially no chance of getting through.
In my system, the PC can then do things like reroll another PC's failed roll, cause a locked door to be unlocked instead, basically mess with timing and probablitily in a way that gives the other heroes that sense of "plot armor/hero's luck" that saves Indiana Jone's neck, and/or establish that the PC had given someone a piece of info, a tool, a trinket, whatever, that just happens to be what they need to get out alive/win the day.
In 5e, it would work with INspiration Dice, Advantage, and a limited pool of "auto-success because PLOT" tokens.