Nails in the coffin

When is a storyline complete? 

Typing "The end" is no guarantee.  To the contrary, that simple phrase becomes a dare, firing the "But what if ..." circuit in authorial brains. Because in fiction, as in life, few things ever truly end.

Okay, the conflict has been resolved. Big deal: life is conflict. Like streetcars, another conflict will be along any minute.

So the Bad Guy has gotten his comeuppance. The world has been saved. Yawn.

You can't keep a good Bad Guy down. He (or she) will always be plotting a comeback. You can't really kill a Bad Guy -- and I don't just mean because of soap-opera-like discoveries that bodies were switched, or the death was faked, or the wrong identical twin bought the farm. We're talking SF, here.

Maybe the corpse can be revived. Maybe a pissed-off clone or a memory upload to the Internet returns to stir things up. Maybe a loop in time or a parallel universe furnishes a just-as-evil copy. Maybe the Bad Guy's robot butler, driver, and cook decide to take over (or battle one another over) the boss's Evil Empire. Maybe ...

Prevent one nasty End of the World, and the universe will cook up another.  The universe is the ultimate Bad Guy that way.

If all that wasn't "The end"-defying enough ... Wait! There's more!

There's that support character with a fascinating personality or a unique point of view. What's her story? What if she should take on a more active role?

And those thousand scraps of background detail, those throwaway bits added to the story merely as color. Any one of them might rear its metaphorical head to demand: How did this bit of tech, or culture, or conflict come about? How might someone use -- or misuse -- this heretofore only mentioned-in-passing capability/vulnerability/quirk? Suddenly, by damn, there's a whole new story in waiting ....

Then there are folks like you, challenging the author, "But what about ...? Just suppose ...? Did you ever consider that ...?"

Hence: sequels. Prequels. Companion novels. Companion series. Reboots.  Re-tellings.  

If flying fingers entering "The end" won't conclude a storyline, what does? Ennui. The cascade ends when the author, or the market, has had enough.

And if the author or the market loses interest in a storyline before you, the reader does? That is a real bummer.

(The cover thumbnails here are from SF novels that became popular series. To be shown does not imply any opinion about the optimal length of these series.) 

Which are your favorite fictional series, SFnal or otherwise? Which series or universe met an untimely end, or continued beyond its expiration date?