Seems like they're preparing a new part of Time Temple.

They don't even ever have to release the Black Mage himself.

He's that one guy in that one story that you never actually encounter, but you encounter small traces of him and you think, ''Oh man, I must be getting closer!'' although you never actually get to him.

Except that's not at all what they've been doing.
They aren't laying break-crumbs; they're laying bricks to build up to the inevitable battle.

There never has been an air of mystery to the character.
We've known since Aran's release exactly what the Black Mage wants: to conquer the Maple World.
We've known since that long what he looks like.
Any back story they've added has been fairly superfluous.

But then The Resistance, KoC, Demonslayer, and the heroes' quests would havn't got any logic anymore if the Black Mage is released, Jerry-Boyle!
The only logical ones would be Adventurers, DualBlade, and Cannonshooter!

The chronology could easily be rectified by using the Temple of Time.
New post-defeat content could be through another door to the future.
A "<x> years later" schtick.

Or, all new content could basically be 'gaiden' stories. That is to say that the Black Mage story would be the end of current chronology (leaving Maplestory 2 to continue it), but they could retcon in more threats that have nothing to do with the Black Mage.