31/1 - Characters' max level is increased to 250!

That's actually easy.

We know, from the tutorial sequence of some classes, that they were level 200 when they fought the Black Mage.
We also know that they could not destroy him. They were barely able, with the help of a Goddess, to seal him.
As we see now, that seal was not strong enough to hold him indefinitely. It's breaking or already broken.
Therefore, obviously, the heroes and all other classes must train harder, and level higher. They must learn new skills, stronger than they ever had before, if they are to defeat the Black Mage when he returns.

As for where these new skills and their energies came from, one idea is that the cooperation and synergy between Maplers and Novans gives rise to powers neither race had before. Or something.

If all fails, apply "A Wizet wizard did it".