Via Siskoid, DC Fifty-Too, in which independent comics folks come up with another 52 DC Comics before the relaunch.
A sideways look at former PCs and lesser-used NPCs could prompt reappearances. What have the survivors of Evil Cult X been up to for the last two years since the PCs blew up their half-manifested god? Is the first supervillain they took down still in jail, planning to break out with a new power upgrade to seek his revenge? What about that guy that dropped out of Wizard School? The street-level vigilantes to your public superhero team? The minor friendly NPCs? Or the PC whose player decided not to play after all?
Once a setting has been around long enough, you could run a one-shot with the players taking the roles of other characters we don't see very often.