Character roster
From Tomb of the Serpent Kings
In Tomb of the Serpent Kings, you don't just play one character. You have a roster of multiple characters you can choose from. Whenever you are going to play, you choose from your stable of PCs and decide which one is going into the dungeon with everyone that session.
This way, you always have a character to play with, even if your other PCs are busy training, or recovering from an injury, or if their level is too low for the current session's goals.
You start with the ability to have up to 3 characters at once. Only one can go on a delve at any time, though.
While at least one of your PCs is at level 3 or higher, you can have a 4th character.