@SebastianAs
@hatakekuro suggested, you should start by creating a Character Sheet so we can start making characters for this game.
If you're looking for a situation that "immediately" brings all the characters together, then you're going to need it to be a HUGE situation. As others have suggested: a tournament, military conflict, or an invasion of some sort.
If you're planning for characters to only join as the lead character finds them, then there can be a unique conflict that can happen specifically for each character that he comes across. This allows the writer of the character he discovers to create a special event to introduce their character.
Either way, in order for the first arc to happen successfully, you need to
cut off the amount of players you are allowing to join your game. Just pause applications until the arc is complete. You have to because people will keep joining the game and you'll never get through them all unless, you go with the first idea where you create a massive event that brings them all together.
If you decide to create a "big event" it should be something that will forever be known in the game's history. There are many games where they make a huge incident and it goes forgotten in the game lore as though it never happened or it wasn't of any big signficance when it was. If this event happens, it should affect the city, region, or even world. Meaning, it could lead to new enemies later or it can be mentioned throughout the game as a reference for something.
