I say, tell players, if you like to chargen and are bored af, make a roster character. If nobody does, don't worry about it. If someone does, you approve them onto the roster if approvable. If someone takes the character but doesn't RP them more than n-amount before dropping them, they go back on the roster. If they get played enough for it to matter, the character is gone same as 'original' characters and it's handled the same way. You also limit the number of times a character can change hands even if it's only one scene per player.